I know BVI Yacht Charters offers 10-day deals in the low seasons, which might lend itself to an orphaned few days between more “standard” charters times. They’re based in Road Town, Tortola, but I think they can organize boats out of USVI as well. They’re very helpful over there, could be worth an email

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There’s nothing, absolutely nothing, half so much worth doing as simply messing about in boats.