the best (cheap) hull material I have come across is metal water pipe, the huge sort they pump water from town to town in. you would have to close off each end but its a good start and round, which means it will be strong.the best way to do buoyancy is to make the craft neutrally buoyant and then push your self down. add two long air tanks at the top, and then add unhappy weight to counter this (just).you now need some air to breath. the best thing to use would be SCUBA tanks. just fill them with air from a compreser and then regulate the airs release. you will need to remove the "stale" air at the same rate that the old air is removed.the best thrust source is an electric motor. I would recommend getting some from old golf carts. you will need to water proof them and add a propeller. put two at the back (forward thrust), two at the front (backward thrust), one either side (left/right thrust), two on top (down thrust), and two on the bottom (up thrust). power them off a golf cart battery would be my advice.to control the motors I would use a set of relays. you could do it like this guy has http://www.homebuiltrovs.com/ (look in the how to section).you could then add alsorts of things like lights, sensors, cameras and manipulator arms.and of course you could paint it yellow.