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There isn't currently a way to seal off blocks or rooms, unfortunately. That does sound interesting, though. I'll have to see how other games handle this and see if I can come up with a way. It probably wouldn't be that difficult to seal them from the top, but I'm not sure how I would handle if you were to open a door or window. That could get tricky. 

I will be adding a bit of physics to when you're underwater, and maybe a ship block / buoyancy block in a future update (similar to how the cockpit works for land / air vehicles). But that's all I've really got planned for water stuff so far. 

Bahh. I was actually just playing around with a moving platform making it look like a boat.

In this genre of games, I typically have seen water handled in one of two ways. One is that it acts like any other  block. For example, if you were to dig a "tunnel" through the water, the blocks would stay in place.

The only other method I can think of is a game like Block Fortress for IOS. If you "break" the last block holding the water back then all the water "fills" in - a static sea level.

Anyway, way over my pay grade. Glad to see your interest is piqued though!