Our familly has visited St. Thomas 3 times and always stop at Coral World. It's a really nice place, and is a doable size without making everyone crazy trying to get from Point A to Point B, etc. All the feeding times are nicely spaced, and it's never been so crowded as to be unpleasant.
My ds (13 at the time) and I did the sea-trekking last time and enjoyed it very much.