Second what Drew said. My first year I used a DX 10.5mm and found it slightly too wide. The following year is was a DX 10-17mm mostly at the 17mm end. Last few years I've gone the DX 12-24mm route mostly at the 24mm end. FX this year and will definitely use the 16-35mm. Gives you the option to go tight on the birds diving or go wide if something big swims in. Fisheye can work but limits your potential shot list. (9 inch Zen dome works fine)
Rgds
Jean.
