Just found this board, so perhaps this St Thomas info is moot. But in the event it's still timely . . .
We've recently discovered cruising after being Caribbean land-lubbers for years. Usually spend a week in St Thomas at Easter break, but was slow in making air arrangements and can't get there (without taking out a second mortgage). Will substitute it with A 5 nighter on Enchantment in April, after having done a weekend quickie on Majesty last month.
Comparing Sapphire, Coki and Trunk Bay on St. John (PLEASE--no S on the end!!) is like asking which is better--Paris, London or Rome? They all have their points and all are gorgeous. We own a timeshare at Sapphire Beach Resort, so clearly we're partial. Let me cover the others first.
Quick mention of Megans Bay. Very scenic, but with very calm water and virtually no snorkling--by our standards--to speak of. I think there's a charge to get it.
Coki--relatively nearby Sapphire, so you might be able to sneak a peek--is very touristy. Busloads of island-based tourists, as well as cruisers, are dropped off continuously. The Coral World underwater aquarium is right next door which contributes to the crowds. The best time I ever had at Coki was when I got up early and was on the beach there before 8AM. Likely impossible from a cruise which is why we had the run of the place at that time. There was a small group of locals doing some water aerobics and that was it. We have our own gear, so we snorkled and fed the fish breakfast They're almost too "trained," looking for handouts the minute you hit the water. You can literally snorkle by bending over at the waist and putting your face in the water. Stayed til 9, and met the first two buses as we left. The place fills in quick. The scenery there is wonderful, but the beach, while long and crescent-shaped, is narrow from tree line to water. If you rent chairs there at the peak, you'll feel like you're on the ship's pool deck on a sunny sea day--packed. They do have a little snack/drink stand there, and snorkle rentals too.
Trunk Bay is also beautiful, with a bit more of a swim to get to the snorkling. But they do have an underwater trail there in that you follow a route, and there are underwater signs identifying some of the fish you'll likely see. Again--can be very crowded. You're also in for the ferry ride from Red Hook which is no big deal, and a nice little adventure, but it does add time to the day (20-30 mins in each direction, plus the cab ride to the beach). Be advised the cabs at the ferry dock are those pick-ups with benches in the back, and they'll wait til they get a full load before starting. A few years ago, they instituted a $5 charge to get into Trunk Bay. Can't recall if that's per head or per car--I think per head. The idea was to cut the crowds, but I'm not sure it's working. Trunk Bay/St John is best visited if you're spending extended time on St. Thomas and you're not rushed in any way.
Sapphire Beach--the jewel in our humble opinion. The beach is the epitome of Caribbean beaches. Crescent-shaped, white sand (the others are too), very wide beach with water that contains no less than 3 or 4 different colored blues and greens. The beach has both full sun and areas shaded by sea grape trees that go right up to the water's edge. There are some places you can have your feet in the water and still be in the shade. This is a beach resort, but you are most welcome here. And because it's a resort there are amenities galore to partake of. As a non-guest you'll have to pay for chairs, mats, wind surfers and paddle boats, but they're there. Two excellent beach bars. One is actually at the pool, which butts up against the beach. Another tremendous sight. The beach is a bit rocky in this area, so head back towards the A-D buildings if you'd rather beach it. This is also where you'd hit the water for snorkling. You'll see the reef under the water, just off the beach. An incredibly easy swim in relatively shallow water. All kinds of coral and fish I can't name except the Parrot Fish. I swear to God I see the same one every year. Technically the pool is for guests, but you might get away with a dip--great after being in the ocean--if you're discreet. We did it for years before breaking down and buying there. The views from Sapphire are of St John, Jost Van Dyke and Tortola. Couple of other smaller islands whose names I don't know can also be seen. And here's the bonus since you said you'll be there on a Sunday. Sapphire Resort has a beach party each and every Sunday, all year round 2-6PM. (Can't speak for Christmas, but I was amazed to see it on our first Easter Sunday down there!) A live calypso/reggae band at the beach bar/pavillion, drink specials and some serious island dancing. You haven't lived til you've "jumped up" with the locals. Who by the way come in droves to this event, dressed in what I'd call casually elegant attire, mixing, mingling abd dancing with us white-bellied tourists in our bathing suits. An absolute blast. Now you'll still have a crowd issue, but there's so much more space. If you're not there early, you may not even be able to rent a chair. Don't sweat it. Just spread a towl under one of the seagrape trees--it'll be worth it.
So head for Sapphire--you won't be sorry. And if all THIS wasn't enough, I'd be glad to answer any e-mails. Enjoy!