You can swim off any number of docks around A.C. The problem is, when you snorkel, you stick your face in the water; ergo, you can't see anything on the surface coming, and they can't see you from the boat, for the most part. Save yourself the worry(and the danger) take a boat trip(about $20.US) to an area that is designated for snorkeling/diving; you'll see more fish, and you're much less likely to get keel-hauled by accident!