1) I am obsessively prepared for trips to the islands 2) I have never missed a flight going or coming.
With that in mind, I would advise trying for the earliest possible ferry from whatever location offers the largest number of options in case there are delays or cancellations. Nothing wrong with getting to St. Thomas early on Sunday and relaxing with a nice breakfast at Hook Line and Sinker or another spot and getting to the airport in plenty of time for the Sunday rush. Beats the heck outta sitting on the ferry dock, praying for a ferry to come along, and hoping that you'll make your flight.
