I went to St Lucia several years ago, and here is my impression.
St Lucia is a spectacularly beautiful island for a vacation. (I stayed at a resort known as "LeSport", I don't even know if it is still there to be honest.) The diving ranged from unimpressive to mediocre. We saw very little fish life and although there were many volcanic formations, there was really very little coral. There were volcanic seamounts, but IIRC they were relatively sterile for lack of a better word.
My recommendation would be that if you are looking for a dive trip, then there are far better places to go and St. Lucia should be put fairly far down you list of dive destinations. If, however you are looking to get away for a week or two in a tropical paradise, then St. Lucia should be near the top of your list.
Keep in mind, my info/opinion is several years old and things might have changed. It is also nothing more than my opinion based on a single trip.