There are a few boats running out of Puerto Del Ray Marina in Fajardo. You also can take the ferry for less than ten dollars each way from Fajardo and just rent a golf cart for the day on Culebra.

http://www.islaculebra.com/puerto-rico/culebra-ferry-schedule.html

http://www.viator.com/tours/San-Juan/Cul...2&aid=g7976

http://www.jerrysjeeprental.com/our-fleet-rates.htm