I haven't been to Jamaica, but my experience with tropical areas is that everything is very casual. You will want to bring at least 2 swimsuits (3 is even better, so you never have to put a wet suit back on), a beach cover-up, some nice shorts and tops sets, a couple of dresses for restaurants and evening wear, a comfortable pair of sandals that can do double duty on the beach and in restaurants, a toothbrush and toothpaste.
If you are going to play any sports (e.g., tennis) bring the appropriate clothing and shoes.
I don't know about Jamaica, but when we went on a catamaran trip in Hawaii the seats were very rough (perhaps so people didn't slide off?) which is very bad for the seat of your swimsuit. So plan on wearing shorts over your swimsuit on the catamaran if you don't want your backside to look like a shaggy dog afterwards.
For restaurants and evening, what you want are long, flowy, sheer type dresses. I like dresses that are not close-fitting as they allow the air to circulate and are more comfortable when walking in the tropics. Be sure to wash the dresses before you bring them to make sure the dyes won't bleed all over you.
My advice is to bring your own sunscreen (the highest SPF you can find, 50 spf is good) and some aloe in case you burn. Be aware that all but natural sunscreens are bad for the coral reefs. If you want to protect the coral reefs, you can bring along this kind of sunscreen:
http://www.amazon.com/Tropical-Sands...rch_detailpage
However, it is not very strong (30spf) and you need to read reviews to learn how to apply it so it doesn't peel off.
Be sure to re-apply sunscreen at least twice as often as you think you need to.
Bring a hat to shade your face, or plan to purchase a hat there.
Again, I don't know about Jamaica, but I would not pack beach towels -- bulky, heavy, and you can usually buy some locally for less money and then leave them behind for the needy.
Bring less makeup than you think you will need. Pack less than you think you will need. Bring *comfortable* sandals to walk on the sand. Be sure to break them in well before you pack them.
My experience is that for snorkeling outings, etc. they will always offer the equipment for rental at a reasonable price. Last year we packed our snorkeling equipment for a trip to Mexico and it was totally unnecessary; we ended up using the boat's equipment for free.
If you have any questions, feel free to call the hotel now. They are the experts who can tell you what to bring, what not to bring, etc.