You won't get a visa in St John, but you can clear in by "scheduled carrier" ie, commercial
aircraft or flight NOT private boat or plane. Make sure your ESTA is in date and carry a copy of it, BVI ferry
companies will ask for it (even though the esta form itself states that it is not necessary to keep a copy!) You will then get a 90 day clearance into the States. This is NOT a "come and go" permit
. Once you leave, technically you have to re-enter. Having said that, its best to ask the advice of the St John Homeland Security
guy who clears you in. Explain that your boat is in Tortola and you want to come back in. They often let you do it - but sometimes they don't!!
Before esta, it was easy and I did it all the time. It should be easier with esta....but it aint! Good luck, Tony