We went to Great Bay beach. The water is clear and warm and calm but it does get deep fairly quickly. Can rent beach chairs and umbrellas (a must) for reasonable cost, many places to eat at and shopping is a very short walk away.
Great port to get a good feel of this country especially old town. Took ferry to Bacardi Rum factory which was nice but took a lot of time (3 hrs), must walk around old town and see Castillo de San Felipe. Good beaches Condado or Ocean Park