I would definitly do Busby for lunch, have a dip and relax a bit, specialy with children, and then go to Captain Oliver's Marina to take the ferry...
Just accross from Busby their is a small Mom & Pop store that sells alcohol, wine, food, snacks etc... Now, not sure about the prices (certainly very high regarding the touristy area it is in..) Otherwise you have another liquor store at Captain Oliver's Marina, just next to the Voyager ferry Office. Or third option, you have 3 superettes in Oyster pond, the closest from the Marina would be "Le Pain d'Epice" where you could buy liquor and wine.