My point was that there is an extensive reef system offshore almost the entire stretch of the beach areas in Punta Cana. It’s too rough to snorkle around the reef and not much gets past the reef. Hence, not many shells or anything to see inshore. Some, but not many of the hotels, are not behind the reef. Their beaches are a little on the rough side. We met a couple a few years ago that were avid snorkelers. They did take the excursions that go to the south coast (twice) and thought the snorkeling was great. Saona Island is also an option, but I hear that there is not much shade to be found there. Whatever you decide, enjoy.
edit: BTW, Saona Island is off the south coast, near Bayahibe. It’s a popular tour destination, and your tour bus picking you up will likely make stops at a lot of hotels, so it’s going to be a long day.