Passenger Ferry
https://www.vinow.com/travel/virgin-islands-ferry-schedules/stjohn/
Car Ferry
https://www.vinow.com/travel/virgin-islands-ferry-schedules/stjohnbarge/
The island is quite mountainous therefore a Jeep Wrangler rental is highly recommended. Also taxis are very limited outside of Cruz bay and some of the northern beaches... so if you plan on exploring the island don't miss out and rent a Wrangler :)
You may rent one on St. Thomas and take the car ferry over... or alternatively take a passenger ferry and rent a car in St. John.
St. John Car Rentals:
https://www.coolbreezecarrental.com
https://courtesycarrental.com/
~The St. John Great House
~Beneath the Mango Tree
~The Tile House
~Villa Arcadia
~Esperance in Fish Bay