Take less of everything than you think you need, apart from spending money.
You can buy any item you might require in the islands and don't need to carry it with you - a couple of shirts, shorts. Reading material is scarce but every bar/restaurant has a trading shelf for books (the Loose Mongoose has the best selection). Your boat will have sheets, pillows and towels and I'd just purchase the toiletries during your provisioning trip. Snorkels and fins are available either on the boat or to take along from the charter company.
A camera, preferably a small waterproof digital one, is a nice-to-have and if you like music you should bring an iPod or MP3 player and hope that your boat's radio
has an input cable for it. If music is a priority then one of those small devices that take the output from an iPod and broadcast it on an FM frequency is nice since every boat radio
will take that as input.
Oh, if you do drink then I'd bring along your favorite analgesic as those are much cheaper in the US than elsewhere.