I've been watching this thread as we're also going on a S. Caribbean cruise (Carnival Destiny) in Aug., and would like to dive/snorkel there.
I'm trying to arrange for our group a combo dive/snorkel trip first, followed by a land tour to see the nearby highlights of the island before getting back on ship.
In St. Lucia, I contacted Scuba St. Lucia in Anse Chastenet. They have a high priced package to include pickup/gears and lunch. The big problem is they insist on a check out shore dive as the first dive, to check out your skills for mask removal and regulator recovery, etc. Their 1-tank boat dive is at 1:45pm. I explained that I have my own gears and I'll be diving in Puerto Rico (La Parquera walls, Desecheo island), USVI St. Thomas, Dominica, Barbados, before going to St. Lucia, and I'm PADI AOWs. But they said no exceptions, everyone goes on a check out dive first. I hear diving at Anse Chastenet is good, but I don't want to spend 1/2 day for the check out dive to be able to go on a 1-tank boat dive. The other killer is we can book but they cannot guarantee the snorkelers will get a snorkel trip because they give priority to resort guests.
I'll look into Dive Fairhelen and others instead, for St. Lucia.