This info is from a boating acquaintance who lives in Vieques. I have not used these services myself.You should verify for yourself and ask your charter company. Alternatively, you might consider planning an itinerary that takes you to Vieques, then back to Puerto del Rey,then to Culebra, or vice versa, if you are convinced you can't make it through a charter without a water refill.
"Christina at 787-930-0831 put in and manages the moorings in Esperanza, Vieques. They are maintained in prime condition. She will also jerry can fuel and water. She has containers if the charterers do not for transport."