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.