Just came back from Turks and Caicos where the water was 76-78 on every dive and I wore a full 3mm. I noticed I was getting slighly chilly towards the end of the dives, but nothing that made me want a thicker suit or ruined the dive. It obviously depends on the diver, there was a guy on the boat who dove in just a skin and everyone renting a wetsuit used 3mm long sleeve shortys (thats all they rent) - I didn't hear anyone complaining about being paricularly cold. Without a wetsuit at all, or just a skin, I would have definitely been freezing ( I am a wuss ). If you're looking to just dive in a bathing suit or skin, then probably a lot of places in the caribbean will be too cold. If you're ok wearing a wetsuit, then I would think you'd be ok most places. I think it gets to the low 70's in the Bahamas so you might need/want a 5mm there.