Bring a couple of bathing suits, shorts, tank tops or tees, a 3 button collar shirt or 2 for dinner and if you are a fancy diner then 1 pair of slacks. Sandals, a hat, sunglasses, sunscreen, bug spray, toiletries and undies....should all fit in a carry on. Snorkel gear if you have it!

I'd add that if you're not really diligent about using good tanning lotion and might get a burn, throw in a light long-sleeved tee or some sort of a cover-up.

Josh-
Remember, less is more! Not only will you bring things down that will end up in your closet all week, but you'll also be bringing gifts/ souvenirs back home. I'd say 2-3 shirts(tees', and 1-2 light button down cabana types-) 1pair of swimming trunks, 2 pairs of underwear, 2 pairs of shorts. Bugspray, tanning lotion/ sunblock, and a pair of comfortable sport type sandals/ flip-flops. You'll most likely have a laundry room available, so that way you're not wearing grubby clothes, not to mention you can clean them before your return home. Poohwear is right on the money.
Smiles-
MJ

One thing that's IMPOSSIBLE to find here, and I always need, are shower thong flip-flops. The ones you find in every dollar store in the States. It's not a good idea to go barefoot on the tile floors, because the hard surface is hard on your back and legs. They're also a good idea to wear IN the shower. So, I'd add shower shoes to the list, and the flip-flops most others are recommending are the more sturdy, street wear type.

Here's the packing list that we use - http://www.garvin.us/STJ/New_Visitors_Guide/STJ_New_Visitor_Guide.html#Packing

I'd consider skipping the long pants.

Cheers, RickG