Hi Lisa!
As for the ferry from STT, we've done it every time but are rethinking things for our next trip. Going directly from the airport to a boat sounds like a win-win if you find the boat you like in St. Thomas. St. John is a beautiful island for stopping.
As for charter companies, we've chartered from TMM lots and love them (disclaimer: we've only chartered from TMM and Virgin Traders so I have no experience with other companies)
As for a cooler, we do it every time. Freeze everything well in vacuum sealed bags. Pack it right before you leave home and don't open the cooler till you're at your boat. Don't bother with dry ice. Use a Polar Bear brand cooler if you can - they're awesome. Check the cooler as a piece of luggage (we fly Delta and have never had a problem with this) When you get to customs in the BVI's, they'll ask you the value of what's in it and you pay 10% tax. I always say $100 and hand them a $10 dollar bill. It's super easy.
Good luck with your planning!
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