I haven't been there in a long time, but there used to be water available near the dinghy landing in Culebra if you didn't mind jugging it back to your boat. Everything is definitely available over in Fajardo, which isn't far away.
The dingy thieves are still active in Vieques. Water is available in Culebra at the Dingy Dock restaurant at .25 / gal. Good news, the old gas station located in the cut has reopened and the too are selling water. This time at .05/gal. Depending on your boat, you might be able to get to the station.