A helpful writeup, Tony, at least for a place to start.
It was on a boat dive I did with DiveTech last year that made me aware of how difficult it is to answer the question of which exposure suit to wear. The water temp was in the low 80s, and we had divers in bathing suits, 3 mm shorties, 3 mm fulls, one 5 mm full (me), and a drysuit! (Tony, you'll know which DM wore that).
I have always worn a 5 mm year round on Cayman (although I just bought a 3 mm full, as it started to get a bit toasty last summer with water temps in high 80s), and the comment I receive the most by some of the other divers is a bit of good-natured ribbing before we get into the water, but then wishing they had a 5 mm by the end of the second dive. As I always say, I'll stop using a wetsuit when the water temp reaches 98.6