We spent a couple of days snorkeling the Fredricksted Pier while on St. Croix. It's on the west end of the island and the water is exceptionally calm there.
If you ever take a trip to St. Croix and plan on spending most of your time in the water then staying on the west end is the thing to do. The only thing you have to watch out for is Sundays. Except for two dive shops, a KFC and a pizza place, EVERYTHING else is closed unless there is a cruise ship in port.
Our first snorkel trip to the pier was on a Sunday and I actually liked it. We probably only saw 10 other people in the whole town! Nice and quiet, just the way I like it!