Try these:
- Navy test site off Diamond Island
- "Empty Pockets" at the same location
- Parker Island wall
- Rum Point
- Ship & Moose Island shoals & wrecks
- Artifacts around any old island with houses
Each of these sites has good viz far below 20 feet, although you'll find most of the best diving throughout the Lake is found in less than 40 feet. The deeper, clearer parts of the Lake offer better viz, and it's always better before the late-summer warm up when the algae blooms wreck everything.