Boats, hard to come by but valuable to have, introduces a source of food (fish obviously).

You would sail the waters carefully looking for random fishing zones that pop up, you would enter the fishing zone and start to fish, this would deplete the fish from the zone, so multiple ships would result in a faster drain and distributing the quota via however many rods are in the water.
This would cause competitive fishing or indeed groups fishing together to take back to their docking bay.
Docked boats are protected (invulnerable and locked) out of fairness, but out in the waters they are vulnerable to piracy (competing fishermen) who can destroy their boats and any fish quota on that ship/boat will transfer to the ship that provided the killing shot.
You could kill the boats occupants but you would then have to steal their boat to drive it back to your own dock to cash in the fish. This will then cause that boat to transfer to the ownership of that dock position. dock bays are assigned by a player agreeing to an unseen force of sharing a portion of the quoata, not providing a reasonable fish share will result in losing the dock bay and it will be freed up for another player.
The payment of fish goes to an unseen (non-physical) company that owns all docks of the island.

This needs work and may be impossible to implement but something that provides risk and reward for using a boat whilest not allowing an unfair advantage would be a good feature.