I have just returned from two weeks in St Lucia. I stayed at the Ladera Resort, which was utterly amazing. I did 13 dives with the Scuba St Lucia dive op, based out of the Anse Chastenet resort (very close to Ladera), and also found them to be very professional and safety conscious.
Water temps were about 82.
The best dive sites on the island are clearly those around the Anse Chastenet resort, which lies within a marine reserve, rather than those around the Sandals resorts in the North. I met several people who made the 1-hour + one-way drive from the Sandals down to Anse Chastenet to go diving.
So, go diving at Anse Chastenet while you are staying at the Ladera, and stick to windsurfing / sailing / whatever while at Sandals.
Getting down from Ladera to Anse Chastenet involves a 20-minute taxi ride plus a 10-minute water taxi, but it is fairly pleasant and you get great views. Ladera provides a complimentary shuttle (once a day). At least on one day, I would stay on at Anse Chastenet to see the sunset there. By the way, the snorkeling at Anse Chastenet is also very nice.
The wreck is indeed called the Lesleen M, and is a very nice dive. Google the Scuba St Lucia home page--they have lots of details.