Try it in a kiddie pool with shallow water. Use POOP floating hulls, but also build weighted stands for each boat. The stands won't hold the boats up, but they will hold the ships in place through friction with the floor of the pool. Terrain (islands, floating fortresses, etc) can be held up on stilts.
Seriously, if you get it to work it would be awesome... especially if you found a way to build watertight submarine hulls. For submarines you could just shorten the stands to drag them underwater, since the weight of the base will ensure that it stays on the floor.