My wife and I stayed at Alibaba's two years ago and just confirmed our stay for next March. You can't get much closer to the beach and snorkeling than here; down about 15 steps, across about 100 feet of sand, and about a 50 yard swim to the reef. Steph and Brian live on site and are warm and helpful hosts. Rooms are basic but clean and sitting on the balcony overlooking the water is about as close to paradise as you can get.
As far as I know Castara Retreat is way up the side of the hill from Big Bay (no snorkeling there) and a good distance from Little Bay.