ROV motor

I think if variable speed is an issue, then you'll be needing another solution, and not one that springs to my mind.

I had envisaged the motors would be turned on to initiate thrust, and when that's been achieved, they'd be turned off. Using two relays, forward and reverse wouldn't be a problem, and using daisy chains of 74HC595 shift registers, 8 relays each, you can control any number of relays, which incidentally, have a UL/C-UL rating of 10A 28VDC / 12A 125VAC.

You can hack this code ( http://www.arduino.cc/en/Tutorial/ShiftOut ) for controlling the relays, substituting the LED's and bypassing the 220ohm resistors.

For a sub, a ballast system would be needed, and if I were lucky enough to be working on such a project, I'd also need to be working on a solution.

Attached to large enough propeller, I think the tooth brush motors would be perfect, they aren't expensive either so you could use a few for thrusting in different directions. Electric screwdrivers would also do the job.

Your battery options are wide open if your using relays, XBee "apparently" works underwater (properly sealed). Maybe Arduino BT could function underwater, if I had one, I'd throw it in a bath tub to test it for you unfortunately I don't, so perhaps someone else might volunteer.

Here's an interesting link with more info on XBee:-

http://blog.faludi.com/category/networking/

The Arduino, I recently discovered, fits nicely inside the old audio cassette boxes, with room for batteries. You'd need to check if there's room for an XBee, fortunately, that also won't cost much to determine.

For water-proofing, silicone sealant doesn't cost much. Though, a few months back I starting working with thermo-plastics (Easycast) which allow you to sculpt a clay model, make silicone (Ultrasil) molds, and fabricate any shape you can imagine in plastic. So, building a custom water-proof enclosure would be a matter of adapting something that's already water-proof, or, you could build one specific to your requirements.

Cameras and control, unfortunately I haven't reached that chapter yet either ;D