I'm sorry, but as a frequent visitor and it's a HUGE pet peeve for myself and residents, it's St. John, not St. John's. St. John's is the capital of Antigua. O.K. now that I've gotten that out of the way...
Getting to St. John isn't really difficult. If you decide to not rent a car, you'll need to take a taxi to the ferry, and then a taxi once you get to St. John to a beach. The entire trip will take about 1 hour if everything works right (from ship to beach on St. John), and up to 2 hours if you just miss ferries, there is traffic, etc. The ferry runs from Red Hook on St. Thomas, every hour on the hour, and the same can be said on the return from St. John. Trunk Bay is the most popular beach on St. John, and it's about a 10 minute taxi ride from the ferry dock. I always recommend those on
DCL to leave Trunk Bay around 1:30 to ensure they'll make the 2PM ferry out of Cruz Bay. This way, if there is significant traffic on St. Thomas (and with a lot of ships in, there always is) you'll not have to worry about getting back to the ship. The 3PM ferry can pull into Red Hook around 3:30, it is know to sometimes run on island time, and with traffic, it can take an hour to return to Charlotte Amalie, trust me I've been in it before. Too much headache for an extra hour IMO. If you get off the ship as soon as it's cleared, usually around 8AM, you can easily make the 9AM ferry out of Red Hook. You'll be sunning on the beach in Trunk by 10AM (probably a little before) and then get a good 3 1/2 hours before you should start heading back.
As I've stated, we're frequent visitors to St. John and usually head down about once a year. It is a wonderful laid back place and a beautiful place to see. If you have any further questions, please feel free to ask.