Seven Mile Beach is a beautiful beach. Crystal clear water and smooth sand. The snorkeling is just so-so because it is just sand without any fish attractors. When you anchor look over and you will see a building with a Diver Down flag. It is Abanks, it is just a short walk from the tender pier. They have a walk down place to snorkel with lots of fish. It is free, they do charge for chairs though. The restaurant has great fritters.
I usually go to Seven Mile then to Abanks to snorkel. Has a great view of the ship, and is close to the tenders as I said so you can stay until you have to leave. Often Grand Cayman is a short port day.
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