The beach there (Klein Bay) is part of Rendezvous Bay on the south shore not too far from Cruz Bay. The location would be convenient to Cruz Bay for groceries, restaurants, etc. The web site of the villa looks fantastic. That pebble beach will likely require some type of shoes to get across to the water, might want to invest in water shoes if you decide to rent there. Haven't snorkeled there but have heard it is good. Since the beach is rocky, I would expect the bottom to be as well which fish seem to prefer.
Good luck.

elagirl-
We stayed at Carnival which is next door to Belle. The snorkeling is sea grass, great for turtles and stingrays, which we saw. The beach is pebble(rock), the view is wonderful. Belle is a private location, nobody to your right and Carnival to your left with plenty of trees/foliage in between. We loved the location.

Tell me a little bit more about Carnival? How big, pool, other amenities? I just found out that Belle Mer has very VERY small rooms, so still looking for something a bit bigger!

elagirl, sorry for the delay. The location, infinity pool, view and beach access to Carnival are fantastic. That being said, Carnival is a one bedroom studio(lower indentical unit also available). OK for a couple-not kids. Have you seen LaMiramar thru Caribbean Villa? It is a 2 br with an "apartment" downstairs to make it 3 and has a little cove off the master for a wee tot . On the water in Chocolate Hole. ROCKY beach-but doable.