The Blue Hole and nearby Rock Lake are both sinkholes, with the water entering from a cave system far below them. With the Blue Hole, we were able to explore that system down to a depth of 200 feet. Rock Lake is about 280 feet deep, but it has no openings big enough to allow entry. The water seeps in through small openings in the rubble left by the collapse of the roof.Depth at BH doesn't matter, as the flow is so high there's effectively no variation in temperature.
That means the water temperature in both lakes is largely determined by the temperature of that deep ground water. The Blue Hole does indeed have a lot of flow because the cave feeding it is relatively large, and the flow goes all the way to the surface. With Rock Lake, the upward flow exits instead through a pipe at about 37 feet, taking the water to the nearby trout hatchery. As a result, there is a layer of water that sits on top and warms up considerably during the summer. When doing a decompression stop at 40 feet, it can be psychologically difficult to be so chilly knowing how much warmer the water is 3 feet above you.
The ground water is actually slightly warmer than the water in the lakes, so some old maps refer to the vents in Rock lake as "warm water vents," leading some people to think there is thermal activity involved, but it is really just the temperature of the ground water.