Battle Royale in a sea doesn't make any sense at all. Circles shrinking shouldn't be a thing. Pirates are supposed to search for riches and make their living, not fighting to the death.
If you noticed in any pirate movies, in the beginning of the movie you would hear any new rumors by any locals that knew any of the urban legend stories, owns a partial map but the map is missing that other players have. They would have to work together or their treasure is lost forever without a completed map. Once they find a treasure together, they have a choice, fight to the death or share the loots. This is a type of pirate it should be made along with 3 factions.
Match-making would go like this: in any of 4 gallons and/or 8 sloops with 24 players would start the game and each outpost has its own urban legend for a certain assignments but needing other players to complete the riddles and/or steal their map in a captain cabin. Finding a treasure would gain experience points, building up the reps. Each server would have their own stories to tell and at end of the assignment, a new treasure needs to be found and players would work together or fight for that specific treasure.
So therefore, the idea of battle royale is overrated in any vision that the developer may have. Fight for one certain loot is worth more exp. points than any skull forts or 3 factions altogether and it should be more of an event once every two hours. Once that event ends, the pirate would go their normal voyage to gain more points but events should be all about working together if they want and could share any exp. points or keep all points to themselves by killing their enemies.
In the beginning of the game, each outpost, a gallons and 2 sloops would get a stories or map that is partial from a local tavern but they are competing against each other from same outpost at the beginning of session but they also need some help from other online player that spawns at different outpost to complete their mission. I think that this type of idea would be huge. Something different than an usual battle royale idea.