Before I make some popcorn and settle in for what should be an ....interesting thread, may I suggest what has been done on a couple of charter boats I saw on the boards?

Each couple puts say $100 in the kitty for things like ice, moorings, water, etc and replenish the fund as needed. If the kitty has anything left at the end of your trip, split it up evenly.

The mooring collection guys will appreciate not having to sit and wait bumping alongside while you discuss who-paid-what-and-when.

Its a VACATION and while I agree frugality is sometimes needed, there should never be a math test as part of the trip.
The other option is for each couple to buy their own drinks ashore and worry about their own 'ship'.
706 jim has sage advice above, cheap is in scarce supply in the BVI.

Also be aware many places will add a 15% service charge whether or not service has been evident, especially with larger groups. We've challenged this charge in several places *cough* Pirates Bight *cough* as the level of service experienced was...well lacking in a large way.
Your results may vary, dealer may sell for less, etc etc.

Oh and enjoy the trip, you've worked really hard to get there!