You'll have to contact them directly and arrange a private charter email@example.com
There's also "No Rush" and they do private family tours firstname.lastname@example.org //ambergriscaye.com/unitytours/
It's best to contact both directly, Unity notes on their website that charters are negotiable. Which probably means you can do a deal especially in low season.
Last edited by SimonB; 05/19/09 04:30 PM. Reason: added info