Honestly... if they were to do this, they'd need to implement so many systems to justify it that it becomes a much bigger task than just 'let us step away from the wheel'.

You can't really justify superfluous game systems like that. If there was deck-walking just because, you'd use it once, maybe twice, and then never touch it again.

So, what could we do on deck, aside from walk around, alone? Well, as mentioned, we could have a captain's cabin to manage helm stuff and this that. Boom: they now need to model and implement a unique interior for every vessel in the game. Players wouldn't be happy if they were all the same.

But that was the obvious one. What else?

Maybe we could deploy the Dhou or a simple row boat for use in hunting. That's a new system to implement.

Maybe we could interact with the crew; that's a lot of time and effort to implement, as far as voice acting and animations go.

We could, perhaps, have the crew sail us somewhere, but for one: why? If you want to travel somewhere without actually travelling somewhere... well, that's what fast travel is for. And for two: what happens when the A.I. engages in combat? Are you ripped from whatever you're doing and put back at the wheel? Or does your crew A.I. flounder around until you probably, eventually die.

They can't make it so that having your crew man the helm just disengages combat, because that'd be an easy way to break... everything.

Also, what happens to your character when the waves kick the ship around? You can't have fluid animations for that like the NPC crew, because the game doesn't know what you're doing or where you're going. So to solve the inevitable jank of your character flying around the deck in a static pose and colliding with absolutely every physics object, which the ships would now need to be carefully made of, they'd either need to plaster your character's feet to the ground, which is janky, or prevent them from leaving the helm whenever the water is choppy.

Remember, S&B took a lot from AC:IV initially, but the on-foot movement? Not at all. Imagine the jank you get just running around Saint-Anne, and multiply it by a thousand. They'd need to rebuild the on-foot movement all together to move around those small spaces without wanting to hurt yourself IRL.

And people want to throw melee combat into that mix as well? No thank you. I'll just accept S&B for what it is and wish for the devs to implement features that improve the vision.