Here are some links to other robotics related websites that I have found useful.
Competitions:
NURC (National Underwater Robotics Challenge) www.h2orobots.org
The tech reports from past competitions are especially good sources of information
NURC archive page | MATE archive page
MATE (Marine Advanced Technology Education) http://www.marinetech.org/
Sources for materials and components:
Mcmaster-Carr – great place to find material, mechanical components, and tools. Good source of O-rings, aluminum, and polycarbonate.
DigiKey, Mouser Electronics, and Jameco – Popular electronic component suppliers. Digikey and Mouser seem to have a much bigger selection, though Jameco has some nice electronics kits for honing your skills.