All around St John the National Parks Service has mooring balls for vessels to hook up to. These balls coast $26 to stay overnight in some of the most perfect anchorages.
Most of the moorings are white and anyone up to 60 foot can use them from a few minutes up to 7 days. Every now and then you will find a blue ball, what do those mean? Well they represent balls that commercial boats have priority on if they have paid $300 for a NPS concession. You can use them without the concession, but you may be asked to vacate the ball if another boat with a concession wants it.
FYI, you are almost always ok to use it overnight, but be prepared to leave by 8am when the day boats start their tours.
A total side story for you…..I was in an empty bay with three balls when a monster boat came in. I asked how big it was and the crew said 90 foot! They were heading to one of the moorings and I informed them that the balls are only rated to 60 foot. The crew flippantly said “we weigh less than most 60 footers”. Ummmm, let me check…..nope it does not say up to the same weight as a 60 footer. It says UP TO 60 FOOT. Some people are just rule breakers!!!!!