I dont see why not. If you were to limit the water to ONLY the top heat spreader portion, and were to completely seal the system (silicon???), then I dont see how it would differ from putting a big chunk of metal on it.
The only thing that could MAYBE be a problem is if you were to electrify the water some how. But I dont see how that would happen, and would probably pose the same risk as running an electric fan on a heatsink (the big chunk of metal).