Good beaches on St. Thomas, Meagans Bay, nice and calm, but no good snorkeling there. If you want to see a beautiful beach, you can take the ferry over to St. Johns, Trunk Bay, great snorkeling and an all around beautiful island. Although it does take some traveling time, you'll need to get a cab to the ferry and another one once you get to St. Johns.
We were on Golden over New Years and in St. Maarten we did the America's cup regatta and it was a lot of fun, you may want to check into that.
You should be able to get into your booking on the Princess web-site and see the list of all the excersions 60 days prior to your cruise, gives you a good idea of what some things you can do.
In St Thomas, take the New Waves Snorkeling excursion. You will get a fun (and I mean fun and fast) ride across to Lesser St James then to St John. We stopped at Honeymoon beach in St John, you could snorkel or go ashore or both.
Last trip to St Martin we skipped the excursions and avoided the crowds by getting in a cab at the terminal (dispatcher will tell you the fare in advance) and going to Dawn Beach. This was a beautiful beach for sunning and swimming and is located in an area on the Dutch side that has condos nearby and some of the nicer houses on the island on the cliffs overlooking the beach. There is a beach bar/restaurant that also has bathrooms. There is also a concession stand that sells food, drinks and rents chairs and umbrellas (they make a great rum punch there).
Arrange with the cab driver to pick you up, or, there are usually cabs there or you can get the bar to call a cab for you.
There were also some really friendly beach vendors selling all the usual
souvenirs at prices the same as in town or better and with no pressure
at all. There is a large Marina adjacent to the beach, we did not go over there to check out what was there.
There must be an offshore coral formation of some sort as there were gorgeous pipeline type breakers a couple of hundred yards out. The shore line was small slow rollers. The beach was frequented by tourists staying on the island, a few locals and we did see 6 or 8 others from our ship.
It was a great beach day.
The best and most seamless swimming/snorkeling/beach opportunity is the "Champagne Catamaran Sailaway to St. John." A sailing catamaran picks up the particpants right at the end of the cruise ship pier and sets sail for some "fun in the sun" at a fairly remote and very pristine beach on St. John. Arriving at the beach, the crew lowers a swimmer's stairway into the water from the bow fo the catamaran for easy access to the water and the beach and breaks out snorkel gear for those who wish to use it. The catamaran remains at anchor for a few hours. While there, the crew serves a buffet lunch aboard the catamaran. The catamaran returns to the pier aruund 3:15 -- which is right in time for tea aboard ship (3:30-4:30) or, if you wish, you can pick up a few souvenirs in Charlotte Amilie before the ship departs. Additionally, the crew serves beverages (your choice -- soda, juices, rum punch, beer, and, of course, champagne, or any combination thereof) aboard the catamaran the whole time. It's one of the more expensive excursions, but well worth the price.