We took a ferry over to Atlantis and then went over to Cabbage Beach. You can walk further down to where there are less people. Right where you walk onto the beach, it is kinda crowded. Beach is really nice. We had our own (cheap) snorkeling stuff, so we just snorkeled around on our own. Water is crystal clear!
There are no facilities here. There are some drink stands and natives come around selling snacks and water (and tee-shirts, necklaces, etc).
Don't try Atlantis - you definitely have to PAY to snorkel on their beaches. But if you WANT to pay about $110 per person they have fantastic facilities including man-made tube rivers with great waterfalls, rapids, etc.