diff --git a/src/water_cmd.cpp b/src/water_cmd.cpp
--- a/src/water_cmd.cpp
+++ b/src/water_cmd.cpp
@@ -794,7 +794,7 @@ void DrawShipDepotSprite(int x, int y, A
 }
 
 
-static uint GetSlopePixelZ_Water(TileIndex tile, uint x, uint y)
+static int GetSlopePixelZ_Water(TileIndex tile, uint x, uint y)
 {
 	int z;
 	Slope tileh = GetTilePixelSlope(tile, &z);
@@ -1202,7 +1202,7 @@ static VehicleEnterTileStatus VehicleEnt
 	return VETSB_CONTINUE;
 }
 
-static CommandCost TerraformTile_Water(TileIndex tile, DoCommandFlag flags, uint z_new, Slope tileh_new)
+static CommandCost TerraformTile_Water(TileIndex tile, DoCommandFlag flags, int z_new, Slope tileh_new)
 {
 	/* Canals can't be terraformed */
 	if (IsWaterTile(tile) && IsCanal(tile)) return_cmd_error(STR_ERROR_MUST_DEMOLISH_CANAL_FIRST);