Water taxi will be the most convenient, but I believe most/all pick you up in Red Hook, not CA (~30 minute ride from airport), which makes for a much shorter boat ride. Ferry is 5-10 minute ride in CA. I believe they are all priced around the same, but perhaps you can find a discount since you are a full (12) boat. We have used Island Time and Dolphin (largest) and have had good results. There may be a surcharge for Scrub as it is a bit further (can't recall but did it last year). If you are willing to pay for it, the Water Taxi is the most enjoyable as you will have it to yourself with drinks aboard, captain checks you in, then drops you off right by your boat. Avoid another cab ride from Road Town, then resort ferry to Scrub.