Regarding the snorkeling beach, are you referring to Grand Cayman? If so, I read that Eden Rock is a short walk to the right of the port building facing the street after you get off of the tender.
We took the Tulum tour years ago and there was a boxed lunch included. There is no time for swimming at this Mexican tender area. You will need to get back to Cozumel. It is about a 30-45 minute ferry ride.