best snorkeling is down around the pitons.. anse cochone, anse chastenet, jelousie beach.
Overnight moorings - Marigot bay (most beautiful spot, hurricane hole), Soufrierie (below the pitons, very nice), Jelousie bay (great snorkeling, good moorings), Rodney bay (can be loud but near lots of activity, hike pigeon island fort, go to town).
Quaint Hotel.. I'd pick something in Marigot Bay. Same for restaurants, quiet bars, etc.
Lot of charters originate in St. Lucia and then go south to Grenada, by passing St. Vincent and hitting the Grenadines. Depending on your time, and what the charter company will let you do, to the north is Martinique, Isle de Saints near Guadeloupe, Dominica. Speaking French can really help if you go to Martinique.