I just learned that you can buy tickets for the St. Thomas to St. John ferries in advance using the miracle of the interwebs.

Just get out your credit card and point your web browser at: St. John Ferry Ticketing Site

You will be e-mailed a receipt with a link to your printable ticket which is in the form of a QR code.

They suggest you should print it out (and I will) but eventually they should be able to read it off your mobile device.

This may not be new but it was new to me so I thought I would share.

I don't honestly know if that's new or not, but most here state that you really should wait until you arrive to buy tickets for the next available ferry. ...and got to say, the concept that they will be able to read your ferry ticket off your mobile device made me laugh...

For ferries to the BVI that was always the advice and was good advice.

For the Red Hook to Cruz Bay run there is only one option right now and the Charlotte Amalie to Cruz Bay ferry appears to not be running due to COVID-19 (not sure how that works but that's a whole 'nother story).

Sorry, I missed that you were talking about only USVI ferries. But e-tickets for the ferry still makes me laugh.

When printed out it also has the passenger name, data, route, and what appears to be a serial number. I suspect they will just take the paper copies.
I laughed, too, when I saw the QR code.

I purchased on line in advance for the Crown Bay/Cruz Bay runs this past July. They only asked for my name when boarding to make sure I was on the "paid" list.

I actually loved the crown bay/cruz bay ferry. Used it in August, much quicker return trip. Got to hang at the marina too, as my flight was delayed by 4 hours while i was on the ferry.... Got in two more dark and stormies !!