Been building an underwater ROV (Remotely Operated Vehicle), for the last few months. Made from plastic plumbing pipe, with onboard electronics for compass, depth, thruster control, lights & video camera.
Its controlled via a laptop on the surface, and connected with a 50m long umbilical cable. Attached a few pictures, working on the final assembly at the moment. But got a little distracted building myself a 20W LED video light (using concepts from my ROV).