Pirates roaming the sea and/or attacking your fortress. Maybe chinese im bamboo-boats. Or vikings in dragonboats. When you kill then to find some extra engrams to craft culture specific stuff. weapons, boats, clothes...
Sry for the Double post but I wanna push this.
But now to my ideas for mods.
- Harbor structures. like in freeports. Maybe some bridges what open up for ships. Like the "Tower Bridge" in London. But not opening by itself. You'll need something to open it. Do it yourself or have a creature what does it for you. Or a kind of lock gate. The door schould be big enough for a Atlas boat