Good source for motors: rechargeable power drills. Small ones can be had for 5-10 euro. They even come with NiCd cells and a charger. I bought some and they have a 3:1 gearbax, wich seems ideal for most boat propellors. They're not waterproof, evidently.
I am building a land prototype first, wich will be wireless. Underwater, you can forget about wireless comms, I think. Hance, GPS will probably not function, unless you are really close to the surface.
Sure, underwater modems exist, but they only offer a few hundred bits/sec...
I really hope someone here will prove me wrong about underwater comms, tho 