I spent two weeks in St. Lucia over the New Years holiday. As far as diving locations/ops, Marigot Bay is a great location. I dove 6 dives with Dive Fair Helen (located in Marigot Bay). Owner Andre is a marine researcher and runs a top flight ops. Also, you are only a 10 minute boat ride to the best southern diving locations.
As far as renting a jeep, do so if you want to do alot of individual exploring. The roads are in decent shape, but they are very winding as the island is extremely moutainous. Also, Marigot Bay is away from most of the dining areas so if you plan on eating out alot it may be cheaper than using taxis. Marigot Bay is physically a stunning area.
If you are interested in excursions, we were very dissapointed with the horseback riding as they take you on country (dirt) roads all the way to the beach, and then you spend more time off the horse at a local beach bar than on.
Tall ship excursion was highly enjoyable (prefered to the catamaran as you get quite a bit more room to roam). Take off from Rodney Bay, sail all the way to the south (great views of the island) decent rum punch, and good tunes. THen you go the the volcano and waterfalls, back to the boat for lunch, snorkeling and sail back. It is a 7 hour cruise and lets you see quite a bit for a decent price.
All in all, we had a great relaxing time in St. Lucia. Once again, I cannot say enough good things about Dive Fair Helen.