The one by the dive school is fab! It's much bigger that the 2 by the pools and a brilliant place to slide into your wetsuit before a dive and warm up/remove wetsuit after a dive! Don't know how it compares to the one on the nude section.........cos I've not been there
I'm pretty sure that the hot tub behind the dive pool is open to anyone. I know we have used it and its never been a problem. Also, we have never needed it to warm up after close to 40 dives at different times of the year. Anyway, dives are usually just twice per day with a occasionsal nght dive, so there would be lots of times no divers would be around. I would say go for it anytime!