Most ship fights end in boarding. When two air bubbles touch, crews can cross from one deck to the other.
Two ships can be boarded when their decks are within 30 feet of each other. Use the chase rules in the Aether Travel and Helms rule to get this close. The Ram action in the Ship Combat rule also works.
Grappling Lines. As an action, a creature can throw a grappling hook or tether line at a ship within 30 feet. It makes a DC 12 Strength (Athletics) check. On a success, the line catches. While at least one line holds, the pilot of the caught ship must win a contested Dexterity check against the other pilot to pull away. Each extra line gives the holding side a +2 bonus.
Cutting a Line. A line has AC 11 and 5 Hit Points. A creature next to it can also cut it as an action with a blade.
Boarding Ramp. Once the ships are locked, a crew can drop a ramp or plank across as an action. Creatures cross a ramp with normal movement.
Ramp or plank. How It Works: Normal movement.
Jump. How It Works: Use the normal Long Jump rules. The gap is usually 10 to 30 feet.
Swing on a line. How It Works: Move up to 30 feet across with a DC 10 Dexterity (Acrobatics) check.
Climb the hull. How It Works: Climbing speed or Grip Boots. Takes 1 round for each 10 feet of hull.
A creature that fails a jump or swing falls into the gap between the ships. Use Going Over the Side in the Air Bubbles and Open Space rule.
Smoke First. Boarding crews often throw smoke canisters onto the enemy deck before they cross, so defenders can't see them coming.
Tight Spaces. Ship halls and cabins are narrow. In a 5-foot-wide hall, only one Medium creature can stand abreast. Weapons with the Heavy property have Disadvantage on attack rolls in a space this tight. Scatterguns and hand cannons are the guns of choice.
Rails and Masts. Creatures behind a rail or mast have Half Cover.
Hatches and Doors. A wooden hatch has AC 15, 20 Hit Points and a Damage Threshold of 5. An iron hatch has AC 19, 40 Hit Points and a Damage Threshold of 10. A barred hatch can be forced open with a DC 20 Strength (Athletics) check.
Player characters fight the officers, monsters and other named foes. The rest of each crew fights in the background.
At the end of each round, each side rolls 1d20. Add 1 for each 5 fighters on that side, and add 2 if a player character Captain or other leader gave an order this round. The side that rolls lower loses 1d4 fighters. On a tie, both sides lose 1.
Breaking. When one side has lost half its fighters, its leader makes a DC 12 Wisdom saving throw. On a failure, that side surrenders or runs back to its own ship. If its leader is down, the side breaks without a save.
A ship is taken when the boarders hold its helm for 1 full round with no defender within 5 feet of it, or when its crew surrenders. A taken ship can be sailed off with its helm key, or its helm can be locked so nobody can sail it.