I think there are 2 reasons you haven't had a rush of answers to your post: water temps are posted somewhere on the AC website and have been posted on the message board here several times in the past. Search around and you'll find the monthly averages. Secondly, everyone feels temperature differently. Or is my family the only one to fight over the thermostat?
Anyway, I get cold easily but I was comfy in my shorty diving the canyons at 40'. You shouldn't need any more protection than that. Just don't forget the sunscreen for the boat ride.
Have a fabulous trip!