I am very familiar with Rock Lake. I will be there this weekend.
The lake is huge in comparison to the the others. I estimate it is about 400 feet across east-west and about 500 feet across north-south. The deepest part is supposed to be 285 feet, but I have not found that yet. The deepest I have been is 275. Like the Blue Hole, it is a sinkhole fed by relatively warm water, meaning that the water is roughly the same temperature year round (about 60° F). The water enters through several openings in the bottom. We have been playing around with opening them some more by pulling out loose rocks and silt, secretly hoping to find something like a cave. (No luck so far.)
It is a typical sinkhole, with steep cliffs and overhangs. At the upper levels, the water plants hang down over the cliffs like the hanging gardens of Babylon. The rock walls are covered in blue/black goethite, so it is very dark. Selenite gypsum crystal clusters have formed in a number of places. Most of them are also covered in goethite, making them purple, but some are still clear crystal. There are also rare freshwater sponges there. I personally think it is one of the most beautiful sites that I dive regularly.
The water exits through a massive pipe that goes to the Rock Lake trout hatchery, at a depth of about 35 feet. This creates a layer starting at that depth that warms up in the summer. The water temperature at that level can get into the 70s then.
The surroundings of the lake are purely natural. The only structures there are a bench for setting up tanks and a open and collapsing tin shed. There is no running water, no electricity, and no plumbing of any kind. I bring a camping toilet. There are wooden steps leading into the water, build many years ago by divers.
The lake is on private property, and entrance is tightly controlled by the owner. Waivers are required, and, of course, a daily entrance fee. You can only get into the lake if you have at least AOW certification and are being supervised (loosely) by one of the instructors who is approved by the owner.
I am one of those instructors. If you are interested in joining me on a trip, let me know. I will be away for 3-4 months after this weekend, but I will be ready to go in the spring.