MerlinDV,
Actually warmth is the main issue! That is the main function of a wet (or dry) suit! Simple physics of heat transfer: your body is a 98.6 deg F and the Caribbean in summer is about 80 deg F; your body will "try" to heat that whole Caribbean and you will get mighty cold! Now, water skiers and swimmers do not notice all that because the sun is warming them and because they get out of the water when they get cold. But you will be down at 90 ft (or whatever) for one hour (or so) and will not be able to get solar heated or climb immediately out.
Now, it is true that you can scuba in the Carib without a wet suit, but most find it more comfortable to use one--it is a matter of sensativity to cold. We have been with divers who wore nothing more than a bathing suit and t-shirt. However, Barbara and I just feel alot more comfortable in a wet suit. We have dived without and noticed the difference!
You can do the experiment and try diving with and without--see if it makes a difference to you. I suspect it will.
One more issue: wet suits do provide some protection against little nasty things with stings: coral, fire worms, jelly fish, etc. In fact, that is why we have our full dive fleeces: we ran into a very large school of thimble-size jelly fish and suffered some stings. Normally coral is not an issue; butif there is surge, you might get bounced against some coral: a wet suit will help you avoid abrasions. Remember, though that touching coral can damage the coral...
And, finally, I have never waterskied and have no idea about how such suits compare to scuba wet suits.
Sorry for the overly long post!
Joewr