I've done Mohonk Lake multiple times. The problem with access is, they only let one person/shop take students to the lake.
The lake is nothing really special. Some fish and BIG trout. A small amount of weeds and a gooey bottom. The max. depth is 61 feet and visability averages about 6-10 feet. A week ago the serface temperature was 82F with the first thermal at 12-14 feet. The temperature dropped to a refreshing 58F.
Close by is Lake Minnewaska in Minnewaska State Park. They allow diving (with buddy only). The rules are pretty simple, out by 16:30 and don't mess up the place. There are nice benches in divers cove so gearing up and entry is easy. This lake is for the most part a dead lake. The pH is low because of the acid rain and the bowl the lake is in is made of shawangunk conglomerate with no buffering capacity. You will see salamanders, water snakes, tadpoles and an interesting rare protein eating plant. (Dont worry it rarely bothers divers.) The visability is 3-12 feet. There is a thermal at abut 12 to 25 feet. The surface temp last week was 78F, but below the thermal at 15 feet it was about 62. Some went to the bottom (72 feet) and found the water to be a brisk 41F.
This year the park has implemented a $20 a year fee to dive in the park, (thank you Geroge Pataki).
Safe diving,
Flea Dog