There are many reputable agencies that handle vills both on the island and with bases in the US or abroad. I recommend you contact all of the agencies with your dates, price range and villa specification to see what is available. Then come back to this or the other SBH forums for further info.
Major villa agencies include (in no order):
St Barth Properties
Ici et La
French Caribbean International
The number of beach villas is limited due to the fact that many beaches are undeveloped. Since the island is only 8 sq miles no place is really far. Snorkeling is not great in but there are a few places. We like Colombier but is is only accessible by a hike or boat ride.
I recommend you find a villa in your price range that supplies the amenities you deem most important and go from there.
Villa shopping is one of the most fun parts of planning a vacation to SBH...at least in my opinion. Have fun and come back with more questions.