The cheapest way to get there is by ferry boat. Fajardo is about an hour away from San Juan. It will cost you $2.25 one way! The thing is that during "high season" (December to August ) you either need to reserve your tickets (almost impossible by phone so you need to go in person to the ticket office) or get to Fajardo before dawn to get your place in line. There are passenger ferries at 9am, 3pm and 7pm also vehicle ferries at 4am and 5pm (some days also at 9:30 am). The most fun, shortest, and easiest way is to catch a 10 passenger bi-plane out of San Juan or out of the E. Coast. But, it can run you up to $90.00 for a round trip ticket (money well spent if you want to avoid the seasonal crowds). If you like peace and quiet, no crowds, empty beaches, etc the best time to go to Culebra is between Sept and Dec but keep in mind that some of the restaurants shut down for a couple of weeks during that period. May is also a nice and "slow" month.