I just returned from 7 days on St John. Here is just a few suggestions for first timers...
Bring- bug spray. the bites were worse than the sunburns
water shoes-several beaches, like leinster, you WILL want them.
snorkel gear-seriously, having your own with you all the time is the best. walmart has some.
toiletries-my friend had to buy shampoo-it was 8.69 a BOTTLE!!!
Water at restauraunts is bottled-expect to pay 2-3.00 each.
Cash-lots of it! Only found one gas station who took credit cards. Only once in 7 days did I find a working atm machine. Also, the power shuts down on the island randomly. You will need cash in all the stores when the credit card machines don't have power.
water cameras-12 to 16.00 each there. bring several with you.
Oh... most important thing I learned, ALWAYS greet the locals with a " Hello. How are you today?" before asking anything! Trust me, we learned the hard way.