The water temperatures at Rock Lake are in the low 60s for most of the year. That is at pretty much at all depths. It is different from the Blue Hole in that the water outlet is a pipe about 35-40 feet deep (I can't remember exactly) pulling the water into the nearby trout hatchery. That means the water entering from below does not exit at the top the way it does at the Blue hole. Consequently, during the spring and summer that top layer does not flow out, and it warms up considerably. By late summer the top 40 feet or so can be in the low 70s. Once you descend past that, it quickly drops down to the low 60s.
Lots of people dive it in 7mm wet suits and hoods. If you want to just rent a suit like that, you should have no trouble. The problem with diving wet suits at that temperature is not the individual dives but rather that you get colder as the day goes on and you can't do as many dives as people in dry suits. If we are diving to 100 feet each time, though, the dives will be shorter, we will have fewer dives per day, and we will have longer surface intervals in the sunshine. Overheating between dives may be more of a problem in the summer, and I know people who will not go there then because of it.
Dry suits are best, though, if you want to dive either in Colorado or New Mexico. You can get as toasty as you want to be by adding more layers of undergarments. In the coldest diving I have ever done (and no, I have not done any ice diving), I overestimated the need, wore too much underwear, and was sweating at the end of a one hour dive. A dry suit class can be arranged, although you may have to order your own dry suit ahead of time, since few shops have rentals on hand. I have a spare one myself that I have loaned out, but you will need to be around 6' tall to use it.
