I generally don't trust the 10am ferry to be reliable. For a 1:30 +/- flight, I'd be on the 7am, have a leisurely breakfast at the Pump Room, casual wander through Charlotte Amalie and then off to the airport.
I'm no fan of being up to that early to catch the 7, but after having the 10 canceled on me and scrambling to find alternative, I find it well worthwhile.