My friend that lives in St. Lucia gave me some tips to check out today. Windjammer landing and Pigeonn Island in the North and Anse Cochon or Anse Chastnet in the south. Haven't had a chance to research these on the web, but intend to do so, since she said there was good snorkeling/diving there. Best wishes!
In addition to cruising through St. Lucia, my wife and I stayed at Windjammer Landing on a fly-in vacation. It's a timeshare/hotel/resort with a lovely, quiet beach.
I can't vouch for the snorkeling there, as I didn't do that on the trip - but the resort is very up-scale, has a nice (pricey) restaurant, and as mentioned, a lovely, quiet beach.