Sounds like the same flight we take all the time (Delta to JFK?). We are always on the 7:30 out of Tortola. Agree with Manpot-it's just so much better to not be stressed. Plus, customs that early is usually a breeze. Our last trip we made it through in less than 15 minutes. You can leave your bags at the taxi stand, walk into town and have a nice breakfast (Glady's is many peoples favorite), do some shopping and make your way to the airport with plenty of time.
The 10AM becomes your backup plan should anything happen with the earlier ferry. It is always wise to have a plan B.