I suggest getting a tropical dry suit... as I always do.
:d
But for wetsuits there are many options that are 5/4/3 suits; torso is 5mm, upper arms/legs are 4mm, lower arms/legs are 3mm. I find this to be a good compromise - usually warm enough, not usually too warm. You can always zip it down a bit if you do get warm. If you get cold easy, this is a good option.
The other option would be a 3mm suit and a 2mm vest (non-zippered) that you can wear under the 3mm suit.
Either of the two choices above would be good first purchases for exposure protection and give you flexibility for future diving.
Maui in January is mid 70's, Cayman Islands are just over 80. You wouldn't think that'd make a difference... but you'd be wrong. Especially if you're doing multiple dives per day over multiple days. I can't imagine diving a 3mm shorty in Maui in the winter.
For Maui... or anywhere, don't discount a hood or beanie to help keep you warmer all-over. "If your feet are cold... put on a hat."
"Cold" is relative, overall and from person-to-person. No one can really tell you what YOU should wear to be comfortable. Do also note that an awful lot of divers seem to confuse "I didn't die of hypothermia" or "I was able to tolerate the cold" with "I was quite comfortable." We dive to have fun, if you're cold... you won't have fun. Personally, I've never ended a dive thinking "Would have been better if I had been cold."
PS - keep in mind that "exposure protection" means more than just thermal protection. A full suit will provide full-body protection against things beyond just getting cold.
---------- Post added January 12th, 2015 at 04:12 PM ----------
I've only gone on my certification dives so far which were at like 40 degrees or cold in a 7mm wetsuit in the Midwest. These will really be my first non-certiication dives. I'm about 5'8" 140 and do get cold a bit easily. Thanks! Also--another question--I just bought a Nikon Coolpix AW120 that's supposed to go to 60Ft and have never used a camera like this before. I see you can buy this float thing for it. Will I need that? Any camera advice would also be welcome. Thanks!
The Coolpix 120 is NOT appropriate for diving. I don't care what they say on the Nikon site... I wouldn't take that camera any deep then the far end of the pool.
As to photography... I strongly suggest waiting until you've got some more dive experience before task loading yourself with a camera. To often we see new divers with cameras crashing. bumping, dragging, crawling over the reef to take a photo.
Personally, I think a diver should be able to hover motionless with ease before ever bringing a camera on a dive. If you need to land on the bottom, hold the reef, or prop yourself against something in order to take a photo... you've got some work to do on buoyancy skills. (Plus, without good dive skills... you're unlikely to get good photos anyway.)