We have been to Grand Cayman twice.
One time we walked to the right when we got off the ship and went to Eden(?) Rock. The first time - great snorkeling - lots of parrot fish - the next time - it was so cloudy we couldn't see anything.
But if you enjoy snorkeling - you really must do the Stringray City tour. We went back to Grand Cayman after we heard so many people rave about that tour. It was awesome - so much so that we are taking a cruise in December just to do Stingray City again. It is located on a Sandbar off the coast - so you must take a tour to get there. We went with Sotos - a great tour - stopped at three places, and had lots of time to visit with the Stingrays. The highlight of our cruise.
By the way - seven mile beach is beautiful and you will have a great time there. Lots of sandy beaches - which means no good snorkeling.
Have a great time