t-shirts & bathing suits (daytime)
polo/surf shirts & shorts (evening wear)
beach towel (you may want to wait & buy this down there)
boat shoes or sneakers
sunglasses (polarized are best)
prescription medication
sunscreen!
books
music CDs
camera & film
snorkel mask & fins (your own will be better than the ones that come with the 
boat - give us your shoe size if you're not bringing your own)
a few zip-lock bags may come in handy for packing wet items
waterproof wallet is good to take along some $ if you decide to swim from 
boat to shore instead of taking dinghy

The Moorings should be able to tell you how to get from the USVI to their 
base on Tortola.  It may be either a combination of ferry to West End (aka 
Soper's Hole) Tortola and then a cab to the base in Road Town or a ferry 
straight to Road Town.  I don't really know.  I'm also no help on how often 
they run or how long they take.  If you can be at the Moorings by 11 or 11:30 
Sunday morning that would be perfect.

Do you have contact info for your hotel in St. Thomas?