Pretty much what the rest have said. A t-shirt a day, 2 pairs of shorts, 3 bathing suits, underware as needed, any meds you need, old ratty tennis shoes and a good sence of humor. This plus the cloths on my back, long pants and a tshirt.
Leave behind any worrie or hurry and relax.
If you know where to look, you can find most anything you need while on island so taking a lot of extra stuff is not necessary
The only thing "special" for diving is heavy booties. The soft sock like kind or full foot fins are not a good idea. You can get pretty much any dive item or service you need on the island. For any dive repair needs, go see Bruce at Caribinn. Extra batteries for your flashlight or camera are also a good idea, rechargables are even better.