Thanks for the answers.
I've loaded the CBP Roam app on my phone, and if the charter co has provided the sticker, i'll try it. (It's US Flagged)
Technology at work...

One crew is a GBR citizen with a Permanent Resident Card, so it should work. If not, an afternoon in Cruz Bay will be fun anyway.

I understand mooring balls may be discounted with my Lifetime National Parks Pass.