For snorkelers that are new to the sport, the Natural Swimming Pool is perfect in that many fish can be seen in shallow clear waters. For more experienced divers, there are many coral reefs and canyons with caves and swim-through holes. Exploring Crab Canyon, you will dive underwater 26 meters through colorful stone arches. Seven Hills has a wall of coral dropping more than 50 meters and Cave Du-Du is a freshwater cave best for advanced divers.
Unfortunately, there is no dive outfitter in Río San Juan [1]. It is best to organize a dive excursion with companies from Sosúa [2].
Links:
[1] http://www.moon.com/destinations/dominican-republic/the-north-coast/rio-san-juan
[2] http://www.moon.com/destinations/dominican-republic/the-north-coast/sosua