Vieques has some really beautiful beaches. There are very nice beaches that you can easily walk to while staying in Esperanza. Other's on the island will require renting a car or hiring a driver. From talking with others there, it sounded like hiring one of the local taxi/collectivo drivers wasn't really such a great idea since they tended to charge a lot more than the car rental just to take you to a beach and them come back an hour or two later to pick you up.
The Esperanza beach is a pretty spot to watch the sunset, and it has some picturesque palm trees leaning out over the narrow strip of soft sand. There's a pier sticking out into the water at one end and we were told there was good snorkeling around the pier pilings. We snorkeled there one afternoon and it really was pretty disappointing. Around the pier and the sailboat moorings just to the right of the pier, we saw a couple interesting things, but overall, there just weren't that many fish to watch in that area.
From Esperanza, its a short walk around the peninsula to get to Sun Beach. Sun beach is a magnificent long cresent of sand. The whole curve feels like it is over a half mile long. It wasn't crowded at all when we were there. Sun Beach Bay is a semi protected bay, but there can be pretty good waves there which would be dangerous for swimming, but we were able to do a little body-surfing there.