As I recall, Soufrieres Bay is right next to the two Pitons of St. Lucia, so it will be a lovely place to anchor. We have snorkeled at the beach between the Pitons, and it was really very charming. I believe it is called Sugar Beach, and is a lovely beach, and the snorkeling was definitely worthwhile. There is also a very nice hotel right there where you might try to get a day pass. I am not sure if they offer that or not, but it would be worth a try.
I am not sure how familiar you are with St. Lucia. They have a lot of attractions around the island, but travel time can be an issue you need to be aware of. We did the hot volcanic mud baths, and that was great fun, but be sure to wear an old bathing suit, as the mud does not easily wash out. Also I have heard there are some nice hiking trails near the Pitons, but we have never been to any of them. All in all, I think it will be a great stop for you.