Red Beach and Blue Beach are definitely my two favorite beach spots in Vieques. These are the beaches the Navy used for recreation before they left Vieques in 2003, and they sure kept the best ones to themselves!
They're similar in shape and look: both are huge crescents of powdery white sand, leading into crystal-clear water that turns shades of turquoise and green as it gets deeper. You can walk out a good few dozen feet before the water gets too deep to stand, and just enjoy the gentle waves lapping around you.
Both beaches are framed by gently rolling green hills in the near distance. You can also see some small, uninhabited islands and outcroppings not too far offshore, which I've never ventured out to, but would like to someday.
Snorkeling here is excellent: you won't get the variety or colors that a coral reef or rocky pier would afford, but you will see stingrays, schools of smaller fish, and possibly even a dolphin if you go out far enough!
In my opinion, Blue Beach is a bit better and worth the extra 5 minutes' drive - it's almost always virtually empty, and even when it's not, you'll be 100 feet from the next person over. Remember to bring food and water to any beaches in Vieques, as there are no refreshment stands around!