In ROAM app, go to the Modes of Travel icon at the bottom of the screen.

Enter vessel make, year, registration country (& state or USCG registered, as applicable), registration number, name of boat, length, CBP User Fee Decal Number, and Flag Country of vessel.

When you submit a ROAM arrival, you start by selecting the vessel that is arriiving and then selecting from your pre-populated list the captain/master and crew/passengers that are on board. You'll then answer online the usual customs declaration questions about which port you are entering, last country visited, and currency and food onboard.