St. John on your own: go via Red Hook
The public ferry to St. John leaves from Red Hook on the island's east end, not from the cruise docks, so the first leg is a taxi across St. Thomas. From there boats leave every hour on the hour from 5:30am to 10pm and reach Cruz Bay in about 20 minutes. The government's posted one-way fare for non-residents is $8.15 for adults.
The hourly sailings are what make a self-guided St. John day work - miss one and the next is never far off - but leave a full hour of slack for the taxi back from Red Hook before all-aboard.
Why the jewellery shops push so hard
The Virgin Islands are a US territory, and US residents get a $1,600 duty-free allowance every 30 days on goods bought here - double the usual $800 from foreign Caribbean ports. Children count, so a family's combined allowance is large. That is the whole economics of Main Street; it is worth knowing the figure before a salesperson quotes it to you.
Sources: dpw.vi.gov, gotostcroix.com, stcroixtourism.com