Don’t take the ferry to Vieques! Though it's cheap at $2 per person, it is not worth the 1.5-hour ride, not to mention that the station is like Ellis Island: a mad exodus of people. Do take the commuter flights for $20 per person. You're in and out of Vieques in 15 minutes and on to the next adventure.
Also, it’s recommended to bring your car on the ferry to Vieques, otherwise you're dependent on cab rides. But keep in mind that the ferry doesn’t ship cars over on the weekends. Cab rides were about $3 per person, which we did. I’m not sure about the car rental availability in Vieques.