You have thought about putting monkeys, people you can rescue and instead they protect you or that there are enemies and when you defeat them they give you styles and with that you can unlock islands, clothes or buy people who help you who are NCPs.
I hope my comment will serve you.
I know it will be a good game.
Have you thought about doing it with other devs that would help you get the game out earlier.
Well, you could add the ability for players to create clans, and make their own ships. You can also add raids on different islands for players to attempt.
In every single game, there must be something the players are constantly working for. , usually the players play a lot to get the item they are looking for; then, just show it off to the server. People like to have unique or special items, so they can be recognized y the other people
Going of RealisticWaves and some other replies you could make a pet system with a different type of pirate-related animals like the parrot, money or maybe even a rat! These could be found while exploring islands or brought from larger towns in shops!
This brings me onto my second concept for your game, adding large islands with villages and towns you could use this picture for inspiration, ideas or an understanding of what I mean! This also means you can add an economy on your game with the ability to make developer products to potentially buy coins.