Begin a boarding combat with two ships that have a gap of 5 feet or less between each other, since a Leap action or a typical gangplank can cover that distance. Ten feet might also work if the boarding party has grappling hooks or other suitable tools. You can leave the ships static on the game map rather than dealing with any ship movement or piloting. For a fun tweak, you can also use two battle maps next to one another—one for each ship—and move one of them 5 or 10 feet at the end each round to mimic their motion on the sea.

Swinging on rigging or grappling hooks is a classic trope and can be a good way to get combat started. Swinging from one ship to another on a rope typically lets a character swing up to double their Speed with a single action if they succeed at a DC 20 Acrobatics check.