Ferry searches should be mandatory. We've ferried to Canada several times (destination Victoria BC) and watched backpackers freely enter and exit, heaven only knowing what was in those backpacks. Also a little frightening driving your car onto one not knowing if another vehicle aboard it is loaded with explosives or weapons. Someone could have skindiving equipment with them and jump overboard and detonate after they have cleared the immediate area. Skindiver training has its possibilities with both ferries and cruise ships. Once on board, they have a captive target group.
Nantucket ferries are being monitored. Some bags searched. Some passengers wanded.