Gosh, Grand Cayman is my favorite spot on the earth.
Yes, you really want a car to get to the best snorkeling places - Cemetary Beach, Smith Cove and to get to the restaurants. The cars are all smaller then in than US cars and its English - they drive on the right - traffic circles are truly confusing!
The Marriot in GCM is on the very south end of 7 mile beach. So you have miles of white sand to walk! But the good snorkeling spots are 3 or 4 miles away .. Goggle snorkel Grand cayman to learn more. And restaurants other than the hotel which has a few including a fun buffet with performers by the sea, are scattered along 7 mile and the south shore - be sure to visit the Grand Old House!