NURC ROV competition and build-your-own ROV
The National Underwater Robotics Challenge [warning: frames, marquees, and other non-standard HTML abound], sponsored by Honeywell Hometown Solutions, is held yearly at Chandler High School in Arizona. Teams of students of almost any age can work together to build a remotely operated vehicle capable of performing some sort of underwater mission -- the 2008 competition (to be held next June) requires the ROVs to work around a crashed plane carrying vials of an important medicine and perform a number of tasks.
In addition to a neat "how-to" page full of cheap ways to design and build subsystems for the ROVs, the organization now sells a complete ROV kit of parts, complete with instruction manual, for $250. Even if you're not part of the competition, this would be a neat thing to explore a backyard swimming pool or lake!
Check out some of the videos of the missions to see what the students have done.