This is just my opinion and suggestion, boarding enemy ships is always a given by attacking players, in most cases the first thing on their mind because of how easy it is to disrupt a crew once you boarded but there should be a sort of penalty for this that way at least they second guess it and maybe try other tactics.
I propose this: The jails in ships basically serve no purpose, much so that the one on the sloop was removed.
What if killing a player that boarded your ship allows a few seconds for you to "capture" the player by pressing x near him when he is dead on your ship thus sending him to your holding cell for a period of time longer than what they spend in the ship of the damned?
There can be an option for him to be sent back to the ship of the damned immediately if he pays a hefty fine or he can wait it out.
But it doesnt have to be a long time it could be maybe 15 seconds or so more than ship of the damned, that way players don't always board your ship as the first thing on their mind.
This can be right after he is killed on your ship and have a second or so while not allowing the player to press y to go to the ship of the damned, if the second passes he then can go to the ship of the damned with no consequence so you have to act quickly.
So its fair for the enemy and fair for you while removing the incentive to automatically board your ship.
For this the cell in the sloop would have to be brought back and the bed slightly moved.
