Keep in mind, if you want to relax on a nice beach fairly close to the port, there is Boatyard beach. They have plenty of chairs, a restaurant/bar and lots of water sports. You can rent wave runners there too.
If it says "catamaran", in St. Lucia, just make sure it is a) a catamaran and b) the seas are not choppy.
We booked a catamaran tour that turned out to be a dirty little motorboat (the bathroom was disgusting, and believe me, I used it)
the seas were too choppy to have gone out on a snorkel tour and I got extremely sick, to the point I had to be hospitalized on the ship when we got back. We made a complaint with Carnival about this tour and we got a refund.
Just book carefully....