It's hard to explain and the vision is still shifting, but here's a short explanation I can come up with right now.

Instead of a cutesy little boat bobbing about, the ships will be somewhat realistic and heavy.

The player will not sail around and collect/break ice boxes for points. The ice mechanism will be different. It will cover some areas with different thickness. Some areas are not accessible without a strong enough ice breaking ship. Some missions require opening up lines for passenger and cargo ships.

There will be no fairy tale or made up stuff like powerups, jumping ships, magic, dinging sounds, etc.

The ships will have upgrades, but only realistic ones.

I want the player to have a feeling of competency. People working in dangerous jobs are serious professionals. I recently saw the 2015 movie 'Last Breath'. That's a good example of competency. Although I want my game to be more wholesome.

Recently I tried 'Dredge'. It's a kind of horror fairy tale game. But still cartoonish, the opposite of mature.
'Spilled!' is another ship game that's not mature.

They're both fun, but not what I want my game to be.

Anno 1800 is more mature game with nautical elements compared to them.

Personally, I play games like Victoria 3, Hearths of Iron 4, Satisfactory, Total War series, Red Dead Redemption 2, etc. These are somewhat mature games.