Jan,

How thick a wetsuit depends on your tolerance.

I'm from Connecticut and my blood isn't as thin as those lucky Floridians.
Last time I was in the Bahamas in the winter, January in my case, I didn't
wear a wetsuit to snorkel. Up on the surface the temperatures aren't too bad.

Diving, you often experience a thermocline. I wore a 3mm long. A tropical
hood isn't a bad idea either. It seriously ups the warmth factor without
taking up much space in your luggage.

Enjoy!

Ellen