If you are doing a ship excursion, they will provide all the snorkeling equipment, but bring along a towel from the ship. Just make sure to return it to the ship, or they will charge you for it.
If you snorkel at Trunk Bay, there is a marked trail underwater that is interesting, plus you can snorkel around a small island with tons of fish, coral, etc. I don't remember any beach chairs, but the towel will do. It is an amazing and beautiful island, well worth the trip over.