Renowned webcomic artist Randall Munroe built a submersible ROV starting with a commercially-available parts kit:

The kit comes with a good set of underwater motors and a sensitive camera, and this summer I started modifying it to use an Arduino and joystick control, running the whole thing over Cat-5 cable (which significantly lightened the tether). This will also let me add other equipment, like a still camera, depth gauge, compass, and sonar.

Randall’s blog cred is pretty well established, by now, but including an Arduino always helps.

Also, check out the zip ties binding the tether cable bundle, above right. They serve a dual function as binary depth markers. The one pictured indicates 14 (1110) meters.

Also2, how cool is it that Randall conducted his initial pressure test in Walden Pond? It’s reportedly the deepest lake in Massachusetts, which I’m sure is why he chose it. But I am slightly geeking out imagining Thoreau stepping through a wormhole in front of his cabin and seeing the xkcd guy out on the Pond, in a canoe, watching live video come up from his homemade robot at the bottom.