If you are planning on sailing in the BVI, you should buy a copy of the Cruising Guide to the Virgin Islands. (There will most likely be a copy on your charter boat.) It will tell you all the places you can anchor, grab a mooring, dump trash, get water or fuel. You should know where you can go for those things even though you may not need them. For example, I’ve had to get water on some trips but not others and I’ve never had to refuel. But I sure wouldn’t want to challenge my sailing skills by trying to sail up to a dock. I’ve done it before but it’s not fun in a 50 foot boat!
In general you will most likely get water and fuel at North Sound on Virgin Gorda or Great Harbour on Jost van Dyke. Make sure you have plenty when you head to Anegada as you can’t fill up there.