Back in 2016, IGN interviewed Mike Chapman about what the game was even was. In the interview (on the second page), when asked about ship customization, Mike revealed that there will be more than just cosmetic upgrades. This includes cannon types, reinforced hulls, or upgraded paddles(which hints towards a rowboat, or the option to row your ship if sailing against the wind, as seen in Pirates of the Caribbean).
While there's hope for at least a bit of vertical progression, he also stated that there will be bigger focus on horizontal progression. In the article there are other things such as this, when asked about enemies: "Chapman’s examples are 'Krakens and pirate skeletons, skeletal lords, sea serpents [and] pirate ghosts.'" The painting canvas, a way of keeping screenshots on your ship, as well as the idea that you have to do a certain task to get off of the Ferry of the Damned are also mentioned.
However, keep in mind that this was almost 2 years ago, so some of these ideas might have been changed or completely dropped.
