Ohhh... NXT's underwater :)
This video shows a maneuverability test for an underwater NXT. The NXT is sealed quite nicely with leads heading out to standard PowerFunctions motors. Apparently there is no problems immersing the motors directly in water, as long as they are thoroughly dried after use to prevent corrosion.
A pair of NXTBees provide communication between a laptop with XBOX 360 controller and the NXT.
Via - http://dexterindustries.com/blog/2012/07/05/underwater-robot-with-the-nxtbee_lego_mindstorms_nxt/