It's at St Thomas, not St Maarten.
They've changed it since the last time we were on the Eastern, so if I make a mistake, someone please correct me.
All passengers must check in before they'll clear the ship for disembarkation.
US citizens just have to show up in the WD theatre for an ID check, they schedule when blocks of cabins have to show up to keep it running as smooth as possible. All members of your party will have to show up. I think groups on excursions get scheduled sooner so they can get organized to get off first.
Non US citizens have a different place to show up and it's a teeny bit more complicated but I don't know the details.