It all depends on if you have to go UNDER water, or your robot can just be like a boat. Maybe there will be some sort of underwater aspect, that if your robot is totally submerged by the end of the round you get 25 points or something along those lines.
If you only need a boat robot, I would make it out of fiberglass. Carve a big piece of styrofoam, coat it with fiberglass, remove foam, bingo, you have a shell. I would make it about 1/8" to 1/4" thick. The reinforce that with aluminum beams so you can ram people
. But then bad times if you ever get flipped over and sink, electronics would go nuts. You could just make the robot like a boat with a sealed top, so it can flip over and still work, kindof like having two boats glued together. But then you need to worry about how to get inside it, how to seal everything, how you're gonna drive yourself... etc. I think it would be major problems for rookies.
As for underwater... not too hard actually. Just need to waterproof the electronics. Otherwise you would have a regular frame, welded aluminum, and fans to power your directions. The tricky part would be to figure out how much boyancy you need to stay at a constant depth. After that, its fun and games.
As for my opinion on which we should have, I say underwater. If you make a boat robot and it sinks, you will not be very happy. Especially if it keeps sinking, every round. But I think people could pull off some really cool themes with boat robots, like have an aircraft carrier or a floating car robot. The change itself to a water arena would be awesome, but I dont think its practical for FIRST. My $.02.