You might check the links at the top for both St. Thomas and St. John under "Beaches" for a comprehensive list of all on both islands.

Personally, Trunk Bay on STJ is a must see. You can catch a 15 min ferry ride to STJ from Red Hook in STT which takes you to Cruz Bay, STJ. There is shopping at Cruz Bay, and then a 10-15 min taxi ride from Cruz Bay to Trunk. On STT, would recommend Coki or Magen's.

I'm sure others have opinions as well....

The furthest reaches on the island from the cruise ship dock are only 25 minutes maximum, so all beaches are in your reach. It just depends on what you are looking for. Magens is rated in the top ten for beauty, but snorkeling is lame there, unless you swim out about 40-50 yards on the North or South ends of the beach. It has all of the amenities, but is the most visited. Coki also has all of the amentities, has great snorkeling, so is probably number 2 on the list of most visited. Sapphire Beach, which is probably number 4 or 5, has everything you are looking for, as well. Secret Harbor would be my personal choice. That's probably number 7 or 8, as far as visited by cruisers, but she's got all you need. Sun, sand, shade, beauty, food, drink, snorkeling...It's a great place and probably won't have more than 20 or 30 people on it at that time.

Thanks a bunch for the tips!!

DEAR CRUISER
IT ALL DEPENDS ON WHAT YOU WANT TO DO....
IF YOU WANT TO SEE A MAGNIFICANT BEACH GO TO MAGENS BAY WHICH IS FAIRLY QUIET. HAS A NICE BAR IF IT IS OPEN IN OFF SEASON?? NICE BUT EXPENSIVE GIFT SHOP. THE SIDES ARE FAIR AT BEST FOR SNORKELING. QUIET WATER TO SNORKEL IN.
IF IT IS FUN AND CARRIBEAN CULTURE YOU ARE LOOKING FOR THEN TRY COKI BEACH. IT IS MORE LIVELY, MORE "BEACH HUTS" FOR COOL BEVERAGES AND SOME FUN LITTLE GIFT SHOPS AND BURGER STANDS. TENDS TO BE CROWDED. HAS GREAT SNORKELING ON BOTH ENDS. NEXT DOOR TO CORAL WORLD.
I USUALLY GO TO BEACH FIRST AND THEN SHOPPING. IT SEEMS TO BE THE OPPOSITE OF WHAT MOST CROWDS DO.
HAVE FUN
SUMER