If one has a US National Parks pass, park moorings @ St. John are half price. That's not a hard way to let your Nat'l Parks pass put its cost back in your pocket.
I was told the park moorings were half price only with a NP Senior card.
I believe, based on the website below, that it is only the Senior Pass (previously called Golden Age) and the Access Pass which give you the 50% off mooring fees. Senior Pass is for 62 and older and Access Pass is for people with disabilities.