I totally agree with Donna- grab a cab at the pier head out to Red Hook Harbor and then grab the water taxi over to St. John Island, there are plenty of beautiful beaches to choose from on St. John Island all a short cab ride from the pier- go to www.vinow.com
for ferry schedules and beach information.
St. Maarten - we took a ships excursion for our beach/snorkel adventure we did the Golden Eagle Catamaran - we loved that. I have heard the Pineal Island has some good snorkeling but i can't give you first-hand info on that.
Curacao- there is a nice beach with decent snorkeling at the Sea Aquarium