I go snorkelling regularly in Juan Dolio. There is a fine coral reef. Unfortunately, depending upon which hotel you are staying at, you may have to go through some water with 10" visibility before getting there. You may be lucky and arrive at a time when the waters are crystal clear and you will have great snorkelling. The area between Costa Caribe and Punta Garza hotels is the best to start from, but the area in front of Decameron has a wonderful reef. Have a great trip. The beach is very nice.
There are excursions to Saona Island, Catalina Island and of course excursions to Santo Domingo and Puerta Plata and Jeep excursions available. You should have a wonderful time.