From a previous post in April:

Originally Posted by picaflores
I traveled to the US last Saturday on the new ferry.
Nice customer service, left on 9:09AM, so pretty much on time.
Cleared customs in St John, after waiting a bit for multiple water taxis to free up the dock for the ferry.
Arrived in Red Hook at 11:10AM, hopped in a shared taxi van, $15/per person to arrive at the airport at 11:40AM.
Hope this is helpful info smile