I get cold very very very easily when I dive. I dove in Belize in March and was very cold with a shorty. However, other people were fine with the shorty and some people did not even wear wet suits. Burr!!! This time I am bringing a full 3 mm.
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.
Harriette Look at //www.ambergriscaye.com/pages/town/town.html