[rumblefish]

Depends on water temps and how often you will be diving, especially on successive days, and to a certain extent, how deep you will be diving.

The more days you dive, the more your core temperature will decrease. After several days diving, you may find yourself getting cold even in warm Carib temps. Next time you're on a dive trip, look at what all the DM's are wearing. Guaranteed that most will be wearing heavy wetsuits.

Water depth can affect the warmth of a suit because the deeper you get, the more your suit will compress. The more compression = less thermo protection.

Given the choice between a shorty and a full, get a full. Besides greater thermal protection, it will also protect you from jelly stings and scrapes that you might get if you accidently brush up against the coral. As for the thickness, it really does depend on your personal succeptibility to the cold. However, I would probably shoot for a 5mm. Why? It's always better to be too warm then too cold. If you find yourself too warm, just open up the neck seal and let some water in to cool yourself down. When you're too cold, there isn't much you can do to warm yourself up other then getting out of the water or putting on an additional layer of protection.