I think you probably feel the same way that I do about getting your Mindstorms electronics close to water. You don’t even want to chance it. But I would love the chance to build a robot that isn’t restricted to movement in two dimensions. I think I could create some really interesting stuff. The whole shape of a submarine robot would be completely different that things I make now. It would also take some calculations to get the buoyancy correct so it doesn’t float or sink too much.
Parts would likely be an issue. You might have to create non-Lego parts for ballast tanks. Not to mention the mostly ineffective propellers and screws that Lego makes.
I was looking around the other day, and I found three videos of people who don’t feel the same way about sinking their Mindstorms in water. It appears that all of these videos were made at institutions of higher learning so the possible loss of high dollar electronics would be covered by the school.
One thing I found really interesting is that in two of these videos, the 9-volt gear motors are fully exposed to the water. Reading some of the comments, the poster says that the motors are almost completely waterproof and very little water enters the housing. I still don’t know if I would take the chance with my own stuff.
(WARNING, REALLY BAD MUSIC!!)