I have a question about ship boarding.
The ships are grappled. The captain of my ship decides to board the other one first.
Now, the battle between the "primaries" starts as a normal combat.
Is my character forced to start the combat on the enemy deck? Can't he choose to stay on its own ship's deck and use ranged attack instead?
after looking over the rules, I think it's purely a DM call. There is nothing the specifies that all pc's have to participate with the combat that "starts" on the ship being boarded.
If I were the DM I would allow a player to remain on the other ship and shoot, but all targets would have at minimum partial cover as per the low wall rules, thanks to rigging and other such things being in the way. This means that while the enemies have partial cover against you, you may or may not also benefit from said partial cover depending on positioning.
A low obstacle (such as a wall no higher than half your height) provides cover, but only to creatures within 30 feet (6 squares) of it. The attacker can ignore the cover if he’s closer to the obstacle than his target.
Unless some in-universe force prevents it, there's no reason whatsoever that your character cannot choose to stay aboard their own ship. Hopefully you have a long range weapon and are good at dealing with cover.