I just came back from there, diving a few of the commercial ones. The water in all 3 was pretty much right on the button at 72. I use a 5mm, and was fine on all the dives, except for ones where we were doing simulated deco, and sitting still for 15 min, I started to get pretty chilled. The last dive (of 8 in all) was for an hr. and a half and I wore a vest also, but not sure if I would have needed it. All of the extended and/or cave divers I saw and have read about go dry. Also adds the extra buoyancy in case of bc failure as you will usually have steel doubles, canister light, and possibly deco bottles.
One other thing, flow can be a big contributor to heat loss also, some springs are almost like a lake, and some have a hundred million gallons of water shooting through them, all clear as glass. They are really quite remarkable to me. Very much like flying.
Tommy