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.

The decks of ships, their rigging, and the motion of the waves can come together to create interesting terrain on the battle grid. Some common terrain effects include the following.

Damaged Decks: Portions of deck that have been damaged by combat or collisions might fall apart when pressure is put on them. When a creature steps on one, they can attempt an Acrobatics checks or Reflex save (typically DC 20) to leap out of the way. If they fall, they can try to Grab an Edge as normal. Falling through usually involves landing belowdecks and needing to reach a ladder or stairs to climb back out.

Flotsam and Jetsam: Debris from a ship or objects thrown overboard can float near a vessel. Characters knocked overboard might use these as platforms to fight from or return to the ship.

Gangplank: Walking the plank requires using the Balance action.

Rigging: Climbing rigging requires a DC 15 Athletics check in most cases. Some rigging features ropes under tension that can let someone move more quickly if they undo or sever a certain knot.

Unstable Decks: The movement of the water or impacts against other ships can shift a deck so it’s harder to traverse. It’s best to use this sparingly, typically no more than every 2 rounds of combat. Declare what the terrain’s effect will be at the start of a round. If the ship is pitched or rolling, decide whether the fore, aft, port, or starboard is elevated. Moving toward that side of the ship is difficult terrain. Extreme movement of the ship on the water can cause the deck of a ship to be uneven ground instead. Since this is a significant drawback and makes combat much more difficult, use uneven ground even less frequently.

Weather: Some of the weather effects described here, which primarily affect sailing combat, could also apply during boarding combat.

Though the focus of boarding combat should be on tactical combat, siege weapons aboard the ships might still come into play. If weapons like ballistae are mounted on the top deck, the crew can use them using the standard siege weapon rules. These weapons can also be disabled as described here.

Ships might attempt to disengage and leave the fight if it’s going badly for their side. The vessel’s pilot can use the Disengage activity to attempt to free the ship. Disengage might not be an option if the ship is securely tethered to another or is otherwise held in place.Disengaging PCs: If the PCs succeed, it’s best to end the encounter and let them escape. If the battle is a climactic encounter in an adventure or campaign, however, you might want to evolve it into sailing combat or a naval chase to keep the drama high.

Disengaging Enemies: Enemies should Disengage only as a last-ditch effort when they’re admitting defeat. If their attempt fails, have them surrender or otherwise accelerate the end of the fight. If the enemies succeed and the PCs truly want to play out the rest of the battle, it’s usually preferable to tweak the details to let the PCs easily catch up and defeat the foes, or to have them narrate how they secure victory. If you expect your players will not accept their foes escaping in defeat, it’s better to have the enemies surrender instead of attempting to Disengage in the first place.