It's possible you might be able to negotiate a deal, but when would you buy airline tickets? That could eat up any savings you might get on booking a
boat last minute.
And it does sound like you need a crewed
charter since you want to learn about sailing. The two options you have are to
charter a boat and get a
captain or to charter a crewed boat which is often all-inclusive.
BVI would probably be a good place to go since it's an easy sailing area with a lot of charter
boats, so odds are probably fair that there could be a last minute deal. The flights are still the biggest issue I see.
As far as
scuba - if that is what you mean, do the first part at home - there's class and pool
work that you don't need to do on
vacation - you can just do the 4 dives you need to finalize the
certification. Snorkeling has not certification and is pretty easy.