Thanks to Make blog I just bumped into this ROV which is made with Lego Mindstorm parts. Looks really cool.
Good inspiration from this video is:
- Number of motors: it has 4 motors working like a quadcopter to keep it from sinking (doesn't seems it has a neutral buoyancy) and to change it's attitude (nose down, nose up), and then 2 motors to move forward and rotate it.
- Propellers: it uses standard boat propellers
- Motors: it also states that motors work fine in water, as long as they are dried after usage
- Communications: it says XBee were used. I'm wondering that is the range of Xbee underwater