I went 1.5-2 hours before departure in the middle of the week in March, and still missed it - I was next in line when the Culebra tickets sold out. I bought Vieques tickets instead, then made a b-line to buy advance tickets to Culebra for the next day. On Sunday, many Puerto Ricans go to Vieques and Culebra as a weekend activity, so you may need to be there even earlier, or try and buy in advance. On Vieques, I recommend going to Sun Bay beach. It's very long and much less crowded than the beaches closer to the ferry dock. On Culebra, Flamenco is the world class beach, but on the flip side it gets much more crowded. It really is amazing though. Zoni beach is another nice one on Culebra, and is practically empty, but in terms of sheer beauty, it doesn't really compare to Flamenco. Renting a golf cart is fun on Culebra, and costs about $40 for the day. If you're going to Zoni, definitely rent one, because the taxi vans don't go there. And even though it really doesn't seem possible when you're at the top looking down, the golf carts CAN make it back up the steep hill before Zoni.