Park by the gate, drop in with your snorkel gear, and swim out under the pier. There's plenty of great stuff to see along the sides by the rocks for those who aren't adventurous enough to go deep, but you will be rewarded if you do. We called this place Turtlelandia. It is unique in that it's a snorkel site in very deep water, so there are big fish out there. And many, many turtles! Best snorkel spot we found on Vieques.