We went to St. Lucia over Thanksgiving a few years ago and stayed at Anse Chastenet. I'm not familiar with the Jalousie Plantation resort. One thing you can expect in November is rain every day, probably several times a day. They tend to be little micro-showers - it rains for a few minutes and blows over. It won't ruin your vacation, but you should be aware of it. After all, the island does have a rain forest.
Diving is good, but different from places like Cozumel and the Caymans. The water is not as clear as those destinations. Coz and the Caymans are coral/limestone islands and have no rivers to carry sediment to the sea, so the water is really clear. St. Lucia is a mountainous volcanic island and has rivers that carry sediment to the sea, so there is more particulate matter in the water. Visibility will max out at about 60 feet.
The other thing that is different about St. Lucia diving is that there are not a lot of larger animals (groupers, jacks, barracuda, eagle rays, green morays, turtles) but there are lots of smaller animals (creole wrasse, damselfish, spotted morays, sharptail eels, fireworms, crinoids, brittle stars). To really enjoy the diving, learn to focus on the little critters. St. Lucia has a great profusion of colorful sponges, particularly the azure vase sponge, but the coral is not as imposing as some other dive destinations. Because it is basically the peak of a volcano, the underwater topography is generally like the surface topography: a steep slope. This is particularly true of the marine park dive sites near the Pitons.
It's generally not considered one of the top dive destinations, but I think the diving there is enjoyable and rewarding.