It's my first time posting here, so please be gentle if that has been suggested already :D
My suggestion: What if there were a few small towns on new, a bit bigger islands? And in between those are merchant- and cargo ships trying to bring goods from one town to another. Taking different routes each time.
Those towns would have Navy soldiers patrolling about, including a Navy base somewhere.
Also at sea are Navy ships patrolling and occasionally protecting the civil ships.
The clue is, that your character gets a bounty for attacking civil ships and towns until your character gets captured or killed by the Navy (?) or your head is brought to the Navy by a player in some way (?). The more damage you cause, the higher is your bounty, and the more the Navy wants your head and actively starts looking for you at sea.
When your bounty is up to a certain amount, it would only decrease a certain amount rather than disappear in case of death/capture.
The navy would do nothing to you as long as you don't have a bounty and don't attack them or civil entities. You can also freely walk in towns, as long as you don't wreak havoc there.
There could also be cool Wanted Posters on outposts and towns.
What do you think?