It is not one other LEGO underwater machine (probably will publish here about that one too, in the near future), recently shown at 'NI Week 2008' in Austin, Texas USA.
But something equally unexpected and amazing!
On the video below two AUVs (Autonomous Underwater Vehicles) performing amazingly.
One submersible and one line follower.
These were Polytechnic University's developed AUVs, shown at ASEE (American Society for Engineering Education) in 2007.
Each AUV here presented, was controlled by an RCX perfectly insulated, to operate underwater. In what concers LEGO motors, they are not watertight but more like water resistant.
Do not submerge your motors at home, because there is an huge chance to damage them. If trying, you are doing it at your own risk!