Its 17.1nm total including the stop in Cruz Bay to clear in (I believe Aguatic clears in there, but you should confirm). Travel time dock to dock will be about an hour. the only unknown is the clear-in time at CBP. Say 30 minutes. So figure about 1.5 hours total. Hope that helps.
I believe Aquatic runs to Red Hook, so add about 30 minutes for the land taxi to STT. I have not used Aquatic before. Hopefully another on TTOL has recent experience they can share with you.
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