I would take the 8:30 am Native Son (via St John to Red Hook). Should be able to get to the airport by 11. I think the 10 am ferry is pushing it. Or you could take the 7 am Smith's Ferry to Charlotte Amalie then stroll the 150 yards over to Frenchtown for a relaxing breakfast before heading to the airport.
We like to take an early ferry, 7 am or 8 am out of West End. On our trip in April, we took the 10 am ferry and only had time to eat a quick lunch upstairs at the ferry dock before heading to the airport at noon. This was on a Wednesday NOT A SATURDAY though.