Underwater Kinetics
http://www.uwkinetics.com/catalog/category/6
has some great underwater lights. Nothing to build.
I own the C4 LED light and it's quite bright. I went over to their factory
in Poway to see the product in more detail. I spoke with a woman about
their light canon. It's a dive light and very bright (450 lumens). The
catalog says it can be used in or out of water.
Anyway, just thought I'd throw another option at you Nate.