Index: src/station_cmd.cpp
===================================================================
--- src/station_cmd.cpp	(revision 27632)
+++ src/station_cmd.cpp	(working copy)
@@ -2504,6 +2504,9 @@
 		return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
 	}
 
+	ret = EnsureNoShipOnDiagDir(tile_cur, ReverseDiagDir(direction));
+	if (ret.Failed()) return ret;
+
 	TileArea dock_area = TileArea(tile + ToTileIndexDiff(_dock_tileoffs_chkaround[direction]),
 			_dock_w_chk[direction], _dock_h_chk[direction]);
 
Index: src/vehicle.cpp
===================================================================
--- src/vehicle.cpp	(revision 27632)
+++ src/vehicle.cpp	(working copy)
@@ -484,6 +484,24 @@
 	return CommandCost();
 }
 
+/**
+* Ensure there is no ship on the water at the given diagonal direction.
+* @param tile Position to examine.
+* @param diag_dir Diagonal direction
+* @return Succeeded command (water is free) or failed command (a ship is found).
+*/
+CommandCost EnsureNoShipOnDiagDir(TileIndex tile, DiagDirection diag_dir)
+{
+	if (HasTileWaterClass(tile)) {
+		TrackBits tb = TrackStatusToTrackBits(GetTileTrackStatus(tile, TRANSPORT_WATER, 0));
+		if (((diag_dir == DIAGDIR_NE && (tb & TRACK_BIT_3WAY_NE) != 0) || (diag_dir == DIAGDIR_SE && (tb & TRACK_BIT_3WAY_SE) != 0) || (diag_dir == DIAGDIR_NW && (tb & TRACK_BIT_3WAY_NW) != 0) || (diag_dir == DIAGDIR_SW && (tb & TRACK_BIT_3WAY_SW) != 0) && !IsShipDepotTile(tile) && IsTileType(tile, MP_WATER) && !IsLock(tile))) {
+			CommandCost ret = EnsureNoVehicleOnGround(tile);
+			if (ret.Failed()) return ret;
+		}
+	}
+	return CommandCost();
+}
+
 /** Procedure called for every vehicle found in tunnel/bridge in the hash map */
 static Vehicle *GetVehicleTunnelBridgeProc(Vehicle *v, void *data)
 {
Index: src/vehicle_func.h
===================================================================
--- src/vehicle_func.h	(revision 27632)
+++ src/vehicle_func.h	(working copy)
@@ -165,6 +165,7 @@
 
 CommandCost EnsureNoVehicleOnGround(TileIndex tile);
 CommandCost EnsureNoTrainOnTrackBits(TileIndex tile, TrackBits track_bits);
+CommandCost EnsureNoShipOnDiagDir(TileIndex tile, DiagDirection diag_dir);
 
 extern VehicleID _new_vehicle_id;
 extern uint16 _returned_refit_capacity;
Index: src/water_cmd.cpp
===================================================================
--- src/water_cmd.cpp	(revision 27632)
+++ src/water_cmd.cpp	(working copy)
@@ -135,6 +135,24 @@
 		cost.AddCost(ret);
 	}
 
+	DiagDirection dir_rotate = axis == AXIS_X ? DIAGDIR_SE : DIAGDIR_NE;
+	TileIndex tc = tile + (axis == AXIS_X ? TileDiffXY(0, -1) : TileDiffXY(1, 0));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
+	tc = tile2 + (axis == AXIS_X ? TileDiffXY(0, -1) : TileDiffXY(1, 0));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
+	dir_rotate = ReverseDiagDir(dir_rotate);
+	tc = tile + (axis == AXIS_X ? TileDiffXY(0, 1) : TileDiffXY(-1, 0));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
+	tc = tile2 + (axis == AXIS_X ? TileDiffXY(0, 1) : TileDiffXY(-1, 0));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
 	if (flags & DC_EXEC) {
 		Depot *depot = new Depot(tile);
 		depot->build_date = _date;
@@ -372,6 +390,25 @@
 	DiagDirection dir = GetInclinedSlopeDirection(GetTileSlope(tile));
 	if (dir == INVALID_DIAGDIR) return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 
+	Axis axis = DiagDirToAxis(dir);
+	DiagDirection dir_rotate = (dir == DIAGDIR_NE || dir == DIAGDIR_NW) ? ChangeDiagDir(dir, DIAGDIRDIFF_90RIGHT) : ChangeDiagDir(dir, DIAGDIRDIFF_90LEFT);
+	TileIndex tc = tile + (axis == AXIS_X ? TileDiffXY(-1, -1) : TileDiffXY(1, -1));
+	CommandCost ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
+	tc = tile + (axis == AXIS_X ? TileDiffXY(1, -1) : TileDiffXY(1, 1));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
+	dir_rotate = ReverseDiagDir(dir_rotate);
+	tc = tile + (axis == AXIS_X ? TileDiffXY(-1, 1) : TileDiffXY(-1, -1));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
+	tc = tile + (axis == AXIS_X ? TileDiffXY(1, 1) : TileDiffXY(-1, 1));
+	ret = EnsureNoShipOnDiagDir(tc, dir_rotate);
+	if (ret.Failed()) return ret;
+
 	return DoBuildLock(tile, dir, flags);
 }