All we wanted to do was Snorkel. And Snorkel we did ... the entire time on shore. No need for a excursion for this if you have basic gear. Take the tender boat to the port piers, walk out the gate. Turn left toward 7 mile beach and within 4 minutes you will come to a beach area and can walk right off the beach to the wreck of the Gamma sticking up out of the water.
Walk half way back and you are at Burger King. Go off right there and this is Cheese Burger Reef. Another nice spot.
But the high light is found at the Eden Rock Dive shop ... just a 4 minute walk in the other direction from the pier. There just 30 yard or so off shore in about 30 foot and less of water is both Edens Rock and the Devils Grotto reef. Great spots.
Note that all beaches in Grand Cayman are public, so as long as there is a public access you can be there. There is much snorkeling just about anywhere, but from what I understand, the best is right there around Georgetown and the closer you get to 7 mile beach the less pleasing the "right-off-the-beach" snorkeling gets.