We went mid March and didn't wear wetsuits.
I almost did because I'm always the first person to be cold, always wear a sweater, etc. But they advised me to try it with a vest first as the wetsuits are hard to take on and off (if you want to take it off for lunch, beach time, etc), and then if I was cold I could always come back and trade for a wetsuit.
Well I never did go back for one. I was perfectly fine. The only time I was a little cold was when I was snorkeling with the fish and just really floating around not moving much. But then I just got out of the water for a bit, warmed up, and went back in later. I wasn't cold for the dolphin swim, and neither was anyone else in our group - they were all just wearing vests too.
I'd say try a vest first and then if you find you're too cold, swap for a wetsuit.