My wife and I have visited St. Lucia in April, 2006. (Early April). We stayed in Rodney Bay with her Brother, but did most diving at or near Anse Chastenet in the south. The weather was barely tolerable, with rain and choppy seas every day. The diving is excellent, however, and had some sunlight for photos.
I would recommend staying at Anse Chastenet, a diver oriented resort that is actually near the international airport in the south. If you choose to stay in Rodney Bay, or elsewhere, beware Castries, the capital and seaport. We have traveled throughout the Caribbean and Mexico in recent years, and Castries is by far on of the worst cities in the Caribbean to visit, drive through, land a cruise ship in, etc.
Many spotted morays, some turtles, no sharks, but I got a rare digital video of a flying gunard (spelling?). Have a great trip!