Underwater light? Why?
As far as what to plant I think others here are more qualified. However I would strongly suggest after you plant your seed to get some Burlap from Dayton Burlap or if you can find it closer, and stake it down over the seed. This will allow the grass to come up through the burlap and the burlap will prevent erosion. The burlap will eventually rot away. When you don't want any wash in of soil that is the way to go.
I think I got a 300 by 6 foot roll for about $100.00.
http://www.daybag.com/industrial/burlap.html#rollsDude you really need to put some trout in there! They are a blast!