You dont get the boat till 6pm?
Well that SUX!! So you will have to spend the night on their dock?
Right, well, they may allow you to scoot around to,a very close anchorage, only a few hundred meters, but still its off their dock. Its at 18°25.508'N 064°36.815'W plot it carefully and you will see its on one side of the channel that leads into the Sunsail, moorings, Footlose dock. But at least you will be off on your adventure.
If you are forced to stay on their dock, or dinghy in, or go before you pick up the boat, there is a large supermarket just across the road and east about 300 meters, in carying distance. You can provision there. And theres a liquor shop on the way back
Do budget for your nights in resturants as they are good fun and part of the BVIs touristy crappy adventure. Foxy's charge like a wounded bull, I think their BBQ is $50 flat plus drinks so its not cheap. And the girls will want a Tshirt and its not a discount store!
So your wallet will be pillaged, but, hey, they are experts in it and you are a 'lil lame tourist.
Cant you fly into Beef Island on tortola and save the taxi and ferry fares?