I have a Henderson Polartex which supposedly is equal to a 3 mil shortly - way too hot! We use Lycra skins over a swim suit, as much for protection from the sun as anything else.
My dive buddy is long and lanky and gets cold easily and was comfortable in a Lycra skin. We have dived there in October and January and the water was about 82 both times.
I wouldn't count on renting anything any place on the island. You would be taking a big chance if you feel you really need something.
Look at http://www.ambergriscaye.com/pages/town/town.html