In Sosua, there are several companies that offer scuba diving and snorkeling excursions. Although it is possible to just swim out to the reef for snorkeling on Sosúa Beach, novices prefer to have a boat for resting. This video from Aqua Center Water Sports and Dive Center shows some of the natural beauty under the water in Sosúa.
If you are new to scuba then I recommend you read about certification and how to plan the trip. Having knowledge beforehand will reduce the amount of time you have to spend learning during your vacation.