diff --git a/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHost.cs b/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHost.cs
index ab9768ceb7..15ea7f37f8 100644
--- a/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHost.cs
+++ b/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHost.cs
@@ -120,15 +120,13 @@ namespace BizHawk.Emulation.Cores.Waterbox
 				data = File.ReadAllBytes(path);
 			}
 
-			var retobj = new ReturnData();
-			NativeImpl.wbx_create_host(nativeOpts, opt.Filename, Reader(new MemoryStream(data, false)), IntPtr.Zero, retobj);
+			NativeImpl.wbx_create_host(nativeOpts, opt.Filename, Reader(new MemoryStream(data, false)), IntPtr.Zero, out var retobj);
 			_nativeHost = retobj.GetDataOrThrow();
 		}
 
 		public IntPtr GetProcAddrOrZero(string entryPoint)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_get_proc_addr_raw(_nativeHost, entryPoint, retobj);
+			NativeImpl.wbx_get_proc_addr_raw(_nativeHost, entryPoint, out var retobj);
 			return retobj.GetDataOrThrow();
 		}
 
@@ -147,22 +145,19 @@ namespace BizHawk.Emulation.Cores.Waterbox
 
 		public IntPtr GetCallbackProcAddr(IntPtr exitPoint, int slot)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_get_callback_addr(_nativeHost, exitPoint, slot, retobj);
+			NativeImpl.wbx_get_callback_addr(_nativeHost, exitPoint, slot, out var retobj);
 			return retobj.GetDataOrThrow();
 		}
 
 		public IntPtr GetCallinProcAddr(IntPtr entryPoint)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_get_callin_addr(_nativeHost, entryPoint, retobj);
+			NativeImpl.wbx_get_callin_addr(_nativeHost, entryPoint, out var retobj);
 			return retobj.GetDataOrThrow();
 		}
 
 		public void Seal()
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_seal(_nativeHost, retobj);
+			NativeImpl.wbx_seal(_nativeHost, out var retobj);
 			retobj.GetDataOrThrow();
 			Console.WriteLine("WaterboxHost Sealed!");
 		}
@@ -174,8 +169,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// <param name="name">the filename that the unmanaged core will access the file by</param>
 		public void AddReadonlyFile(byte[] data, string name)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_mount_file(_nativeHost, name, Reader(new MemoryStream(data, false)), IntPtr.Zero, false, retobj);
+			NativeImpl.wbx_mount_file(_nativeHost, name, Reader(new MemoryStream(data, false)), IntPtr.Zero, false, out var retobj);
 			retobj.GetDataOrThrow();
 		}
 
@@ -185,8 +179,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// </summary>
 		public void RemoveReadonlyFile(string name)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_unmount_file(_nativeHost, name, null, IntPtr.Zero, retobj);
+			NativeImpl.wbx_unmount_file(_nativeHost, name, null, IntPtr.Zero, out var retobj);
 			retobj.GetDataOrThrow();
 		}
 
@@ -196,8 +189,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// </summary>
 		public void AddTransientFile(byte[] data, string name)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_mount_file(_nativeHost, name, Reader(new MemoryStream(data, false)), IntPtr.Zero, true, retobj);
+			NativeImpl.wbx_mount_file(_nativeHost, name, Reader(new MemoryStream(data, false)), IntPtr.Zero, true, out var retobj);
 			retobj.GetDataOrThrow();
 		}
 
@@ -207,9 +199,8 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// <returns>The state of the file when it was removed</returns>
 		public byte[] RemoveTransientFile(string name)
 		{
-			var retobj = new ReturnData();
 			var ms = new MemoryStream();
-			NativeImpl.wbx_unmount_file(_nativeHost, name, Writer(ms), IntPtr.Zero, retobj);
+			NativeImpl.wbx_unmount_file(_nativeHost, name, Writer(ms), IntPtr.Zero, out var retobj);
 			retobj.GetDataOrThrow();
 			return ms.ToArray();
 		}
@@ -262,13 +253,13 @@ namespace BizHawk.Emulation.Cores.Waterbox
 
 		private class WaterboxPagesDomain : MemoryDomain
 		{
-			protected readonly WaterboxHost _host;
+			private readonly WaterboxHost _host;
+
 			public WaterboxPagesDomain(WaterboxHost host)
 			{
 				_host = host;
 
-				var retobj = new ReturnData();
-				NativeImpl.wbx_get_page_len(_host._nativeHost, retobj);
+				NativeImpl.wbx_get_page_len(_host._nativeHost, out var retobj);
 
 				Name = "Waterbox PageData";
 				Size = (long)retobj.GetDataOrThrow();
@@ -279,8 +270,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 
 			public override byte PeekByte(long addr)
 			{
-				var retobj = new ReturnData();
-				NativeImpl.wbx_get_page_data(_host._nativeHost, Z.SU(addr), retobj);
+				NativeImpl.wbx_get_page_data(_host._nativeHost, Z.SU(addr), out var retobj);
 				return (byte)retobj.GetDataOrThrow();
 			}
 
@@ -292,15 +282,13 @@ namespace BizHawk.Emulation.Cores.Waterbox
 
 		public void SaveStateBinary(BinaryWriter bw)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_save_state(_nativeHost, Writer(bw.BaseStream), IntPtr.Zero, retobj);
+			NativeImpl.wbx_save_state(_nativeHost, Writer(bw.BaseStream), IntPtr.Zero, out var retobj);
 			retobj.GetDataOrThrow();
 		}
 
 		public void LoadStateBinary(BinaryReader br)
 		{
-			var retobj = new ReturnData();
-			NativeImpl.wbx_load_state(_nativeHost, Reader(br.BaseStream), IntPtr.Zero, retobj);
+			NativeImpl.wbx_load_state(_nativeHost, Reader(br.BaseStream), IntPtr.Zero, out var retobj);
 			retobj.GetDataOrThrow();
 		}
 
@@ -308,8 +296,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		{
 			if (_enterCount == 0)
 			{
-				var retobj = new ReturnData();
-				NativeImpl.wbx_activate_host(_nativeHost, retobj);
+				NativeImpl.wbx_activate_host(_nativeHost, out var retobj);
 				retobj.GetDataOrThrow();
 			}
 			_enterCount++;
@@ -317,16 +304,16 @@ namespace BizHawk.Emulation.Cores.Waterbox
 
 		public void Exit()
 		{
-			if (_enterCount <= 0)
+			switch (_enterCount)
 			{
-				throw new InvalidOperationException();
-			}
-			else if (_enterCount == 1)
-			{
-				var retobj = new ReturnData();
-				NativeImpl.wbx_deactivate_host(_nativeHost, retobj);
-				retobj.GetDataOrThrow();
+				case <= 0:
+					throw new InvalidOperationException();
+				case 1:
+					NativeImpl.wbx_deactivate_host(_nativeHost, out var retobj);
+					retobj.GetDataOrThrow();
+					break;
 			}
+
 			_enterCount--;
 		}
 
@@ -334,13 +321,12 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		{
 			if (_nativeHost != IntPtr.Zero)
 			{
-				var retobj = new ReturnData();
 				if (_enterCount != 0)
 				{
-					NativeImpl.wbx_deactivate_host(_nativeHost, retobj);
+					NativeImpl.wbx_deactivate_host(_nativeHost, out _);
 					Console.Error.WriteLine("Warn: Disposed of WaterboxHost which was active");
 				}
-				NativeImpl.wbx_destroy_host(_nativeHost, retobj);
+				NativeImpl.wbx_destroy_host(_nativeHost, out _);
 				_enterCount = 0;
 				_nativeHost = IntPtr.Zero;
 				GC.SuppressFinalize(this);
diff --git a/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHostNative.cs b/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHostNative.cs
index 46e7b59070..75ef24948c 100644
--- a/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHostNative.cs
+++ b/src/BizHawk.Emulation.Cores/Waterbox/WaterboxHostNative.cs
@@ -6,21 +6,19 @@ using BizHawk.Common;
 
 namespace BizHawk.Emulation.Cores.Waterbox
 {
-	public unsafe abstract class WaterboxHostNative
+	public abstract class WaterboxHostNative
 	{
-		[StructLayout(LayoutKind.Explicit)]
-		public class ReturnData
+		[StructLayout(LayoutKind.Sequential)]
+		public unsafe struct ReturnData
 		{
-			[FieldOffset(0)]
-			public byte ErrorMessageStart;
-			[FieldOffset(1024)]
+			public fixed byte ErrorMessage[1024];
 			public IntPtr Data;
 
 			public IntPtr GetDataOrThrow()
 			{
-				if (ErrorMessageStart != 0)
+				if (ErrorMessage[0] != 0)
 				{
-					fixed(byte* p = &ErrorMessageStart)
+					fixed (byte* p = ErrorMessage)
 						throw new InvalidOperationException(Mershul.PtrToStringUtf8((IntPtr)p));
 				}
 				return Data;
@@ -73,15 +71,15 @@ namespace BizHawk.Emulation.Cores.Waterbox
 
 		// public delegate UIntPtr /*MissingFileResult*/ FileCallback(IntPtr userdata, UIntPtr /*string*/ name);
 
-		public static WriteCallback MakeCallbackForWriter(Stream s)
+		public static unsafe WriteCallback MakeCallbackForWriter(Stream s)
 		{
 			var ss = SpanStream.GetOrBuild(s);
-			return (_unused, data, size) =>
+			return (_, data, size) =>
 			{
 				try
 				{
 					var count = (int)size;
-					ss.Write(new ReadOnlySpan<byte>((void*)data, (int)size));
+					ss.Write(new((void*)data, (int)size));
 					return 0;
 				}
 				catch
@@ -90,15 +88,15 @@ namespace BizHawk.Emulation.Cores.Waterbox
 				}
 			};
 		}
-		public static ReadCallback MakeCallbackForReader(Stream s)
+		public static unsafe ReadCallback MakeCallbackForReader(Stream s)
 		{
 			var ss = SpanStream.GetOrBuild(s);
-			return (_unused, data, size) =>
+			return (_, data, size) =>
 			{
 				try
 				{
 					var count = (int)size;
-					var n = ss.Read(new Span<byte>((void*)data, count));
+					var n = ss.Read(new((void*)data, count));
 					return Z.SS(n);
 				}
 				catch
@@ -124,13 +122,13 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// which will not be used after this call.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_create_host(MemoryLayoutTemplate layout, string moduleName, ReadCallback wbx, IntPtr userdata, ReturnData /*WaterboxHost*/ ret);
+		public abstract void wbx_create_host(MemoryLayoutTemplate layout, string moduleName, ReadCallback wbx, IntPtr userdata, out ReturnData /*WaterboxHost*/ ret);
 
 		/// <summary>
 		/// Tear down a host environment.  If called while the environment is active, will deactivate it first.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_destroy_host(IntPtr /*WaterboxHost*/ obj, ReturnData /*void*/ ret);
+		public abstract void wbx_destroy_host(IntPtr /*WaterboxHost*/ obj, out ReturnData /*void*/ ret);
 
 		/// <summary>
 		/// Activate a host environment.  This swaps it into memory and makes it available for use.
@@ -139,21 +137,21 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// Ignored if host is already active.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_activate_host(IntPtr /*WaterboxHost*/ obj, ReturnData /*void*/ ret);
+		public abstract void wbx_activate_host(IntPtr /*WaterboxHost*/ obj, out ReturnData /*void*/ ret);
 
 		/// <summary>
 		/// Deactivates a host environment, and releases the mutex.
 		/// Ignored if host is not active
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_deactivate_host(IntPtr /*WaterboxHost*/ obj, ReturnData /*void*/ ret);
+		public abstract void wbx_deactivate_host(IntPtr /*WaterboxHost*/ obj, out ReturnData /*void*/ ret);
 		/// <summary>
 		/// Returns a thunk suitable for calling an exported function from the guest executable.  This pointer is only valid
 		/// while the host is active.  A missing proc is not an error and simply returns 0.  The guest function must be,
 		/// and the returned callback will be, sysv abi, and will only pass up to 6 int/ptr args and no other arg types.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_get_proc_addr(IntPtr /*WaterboxHost*/ obj, string name, ReturnData /*UIntPtr*/ ret);
+		public abstract void wbx_get_proc_addr(IntPtr /*WaterboxHost*/ obj, string name, out ReturnData /*UIntPtr*/ ret);
 		/// <summary>
 		/// Returns a thunk suitable for calling an arbitrary entry point into the guest executable.  This pointer is only valid
 		/// while the host is active.  wbx_get_proc_addr already calls this internally on pointers it returns, so this call is
@@ -161,14 +159,14 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// a pointer to another function).
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_get_callin_addr(IntPtr /*WaterboxHost*/ obj, IntPtr ptr, ReturnData /*UIntPtr*/ ret);
+		public abstract void wbx_get_callin_addr(IntPtr /*WaterboxHost*/ obj, IntPtr ptr, out ReturnData /*UIntPtr*/ ret);
 		/// <summary>
 		/// Returns the raw address of a function exported from the guest.  `wbx_get_proc_addr()` is equivalent to
 		/// `wbx_get_callin_addr(wbx_get_proc_addr_raw()).  Most things should not use this directly, as the returned
 		/// pointer will not have proper stack hygiene and will crash on syscalls from the guest.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_get_proc_addr_raw(IntPtr /*WaterboxHost*/ obj, string name, ReturnData /*UIntPtr*/ ret);
+		public abstract void wbx_get_proc_addr_raw(IntPtr /*WaterboxHost*/ obj, string name, out ReturnData /*UIntPtr*/ ret);
 		/// <summary>
 		/// Returns a function pointer suitable for passing to the guest to allow it to call back while active.
 		/// Slot number is an integer that is used to keep pointers consistent across runs:  If the host is loaded
@@ -177,12 +175,12 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// The returned thunk will be, and the callback must be, sysv abi and will only pass up to 6 int/ptr args and no other arg types.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_get_callback_addr(IntPtr /*WaterboxHost*/ obj, IntPtr callback, int slot, ReturnData /*UIntPtr*/ ret);
+		public abstract void wbx_get_callback_addr(IntPtr /*WaterboxHost*/ obj, IntPtr callback, int slot, out ReturnData /*UIntPtr*/ ret);
 		/// <summary>
 		/// Calls the seal operation, which is a one time action that prepares the host to save states.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_seal(IntPtr /*WaterboxHost*/ obj, ReturnData /*void*/ ret);
+		public abstract void wbx_seal(IntPtr /*WaterboxHost*/ obj, out ReturnData /*void*/ ret);
 
 		/// <summary>
 		/// Mounts a file in the environment.  All data will be immediately consumed from the reader, which will not be used after this call.
@@ -190,7 +188,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// in every savestate, or never appear in any savestates.  All savestateable files must be added in the same order for every run.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_mount_file(IntPtr /*WaterboxHost*/ obj, string name, ReadCallback reader, IntPtr userdata, bool writable, ReturnData /*void*/ ret);
+		public abstract void wbx_mount_file(IntPtr /*WaterboxHost*/ obj, string name, ReadCallback reader, IntPtr userdata, bool writable, out ReturnData /*void*/ ret);
 
 		/// <summary>
 		/// Remove a file previously added.  Writer is optional; if provided, the contents of the file at time of removal will be dumped to it.
@@ -198,7 +196,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// If the file has been used in savestates, it does not make sense to remove it here, but nothing will stop you.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_unmount_file(IntPtr /*WaterboxHost*/ obj, string name, WriteCallback writer, IntPtr userdata, ReturnData /*void*/ ret);
+		public abstract void wbx_unmount_file(IntPtr /*WaterboxHost*/ obj, string name, WriteCallback writer, IntPtr userdata, out ReturnData /*void*/ ret);
 
 #if false
 		/// <summary>
@@ -217,7 +215,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// Must always be called with the same sequence and contents of readonly files.
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_save_state(IntPtr /*WaterboxHost*/ obj, WriteCallback writer, IntPtr userdata, ReturnData /*void*/ ret);
+		public abstract void wbx_save_state(IntPtr /*WaterboxHost*/ obj, WriteCallback writer, IntPtr userdata, out ReturnData /*void*/ ret);
 
 		/// <summary>
 		/// Load state.  Must not be called before seal.  Must not be called with any writable files mounted.
@@ -226,7 +224,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// Errors generally poison the environment; sorry!
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_load_state(IntPtr /*WaterboxHost*/ obj, ReadCallback reader, IntPtr userdata, ReturnData /*void*/ ret);
+		public abstract void wbx_load_state(IntPtr /*WaterboxHost*/ obj, ReadCallback reader, IntPtr userdata, out ReturnData /*void*/ ret);
 
 		/// <summary>
 		/// Control whether the host automatically evicts blocks from memory when they are not active.  For the best performance,
@@ -240,7 +238,7 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// Retrieve the number of pages of guest memory that this host is tracking
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_get_page_len(IntPtr /*WaterboxHost*/ obj, ReturnData /*UIntPtr*/ ret);
+		public abstract void wbx_get_page_len(IntPtr /*WaterboxHost*/ obj, out ReturnData /*UIntPtr*/ ret);
 
 		/// <summary>
 		/// Retrieve basic information for a tracked guest page.  Index should be in 0..wbx_get_page_len().
@@ -253,6 +251,6 @@ namespace BizHawk.Emulation.Cores.Waterbox
 		/// 0x80 - dirty
 		/// </summary>
 		[BizImport(CallingConvention.Cdecl)]
-		public abstract void wbx_get_page_data(IntPtr /*WaterboxHost*/ obj, UIntPtr index, ReturnData /*byte*/ ret);
+		public abstract void wbx_get_page_data(IntPtr /*WaterboxHost*/ obj, UIntPtr index, out ReturnData /*byte*/ ret);
 	}
 }