Well, I was finally able to find someone to dive with me in Green Lake (17 September 2011). The water temps were about 70 at the surface & dropped to the mid 60’s below the thermal. We were wearing full 7mm suits with hoods & gloves, so the water was really quite comfortable. The issue was the visibility.
Below about 5 feet, vis dropped to less than 3 feet and at 30 feet, where I encountered our boat’s anchor, vis was about 1-2 feet. Very much like swimming in diluted milk - but not "silty" like a quarry. All-in-all, vis was so poor that my buddy and I kept losing each other unless were literally touching. After about 10 minutes, we aborted the dive and just enjoyed some afternoon sun on the lake while our gear dried.
I did talk to one other diver that has been in the lake and he told me he dove the lake in late spring/early summer and vis wasn’t too bad. He described the bottom as a "desert" - nothing but sand and the occasional barrel that has sunk (likely from swim platforms). Other than that, nothing to see :(.