One warning regarding all inclusive resort scuba diving - Sandals is my experience. The resort was awesome and we had a great time but the diving was not great. On average about 30 people on the boat with most dives not lasting longer than 35 minutes. Also take a Doctors note if you have any issues as they make you fill out the medical questionairre to dive, not just for a course.
If you want serious diving then avoid them.
Some info on Sandals St Lucia:
We stayed at the Sandals in St Lucia this September - The Halcyon. It was awesome. Lots to do, great food, awesome pools etc. I would stay at the Regency though. It had all the best restuarants and we spent most evenings in the Buses going there for dinner.
They have a 3 in 1 option. Meaning you can stay at any of the 3 hotels and there are buses every hour to take you to any of them. You can eat and use facilities at any of them. So that is pretty cool.
Also the actual diving is done from the harbour not straight from the resort so it is an extra bus ride to go diving each morning.
As before we had some great dives which we enjoyed but I wouldn't stay at Sandals if I were going for a dive trip. The Island is also really beautiful and the people are all really friendly. Try get up the Pitons, we used a guy called Joe. He was awesome.
Joe Knows Tours: Cell: (758) 717-1893, Home: (758) 450-3847
Don't really have any ideas on other places to stay if you want more serious diving but friends of ours stayed at Tikaye where they were basically the only people on the boat that time of year. They said the diving was great but the place was really quiet.
Some pictures of the diving and the sandals resorts from our trip:
http://photo.wildtree.net/scubadude/