We have looked at water taxis before as well and were getting similar costs as Momx3. The water taxi estimates I was getting approach the same cost as a private charter flight.

We elected to use Fly BVI instead which cost us $750 for four people one way. You leave as soon as you get your luggage as your pilot takes you right to the plane and off you go to Beef Island. $12pp taxi from Beef to Roadtown and you are there in no time flat. Have gotten from STT to the boat in just over an hour from landing before. Also don't have to worry about customs line when you are the only plane arriving.

Just another option if you are going to splurge on something other than the public ferry, which is certainly the cheapest option.