We went in December,2008. Diving was so-so. We stayed at Anse Chastanet and dived with their dive operator, Scuba St. Lucia. They're organized and dive boats were ok. Nitrox was offered for an extra fee.
Your first dive is a shore dive, which is a checkout dive - mask removal and putting it back on underwater. Dive sites are all within a 5-10 minutes boat ride. Visibility was not as good in the winter, when we were there. A couple of divers,who'd been there before, said that the diving is so different in the summers with better visibility. They do offer night dive 2-3 times a week. We did one and this was a shore dive - right in front of the dive shop, where you also do your checkout dive.
Here is a link to a few pictures I took; copy and paste it in your browser to work:
Picasa Web Albums - Tita
As you can see, I did not take that many photos. I've been to Bonaire, Grand Turk, Little Cayman....just to give you an idea of where I am coming from. I thought the diving in St. Lucia was so-so; although another couple we met loved the place and been returning there the last 5 years.
Another dive operation that several divers mentioned is Dive Fair Helen. I don't know much about them but you can probably google them. Hope this helps.