I think wearing a wet suit has more benefits than just keeping you warm. My husband and I wore 3mil wetsuits snorkeling - for us it was nice for the bouyancy factor and to keep the sun off of our backs. I certainly wasn't hot but the other two people on the boat wearing shorts and T-shirts didn't seem too cold either.
We also already had them, so it wasn't like we had to rent them special. I always seem to get cold diving - whether it's up here in Wisconsin or in Mexico or Belize. So I wore the 3mil full suit doing 2 local dives one day and 3pack Blue Hole dive the next. I even wore a hood on the 3-dive day, but hey, I'm a wimp that would rather be toasty than a bit chilly.