While I would welcome support for ships and ship battles in this game, recall that this is abit before the use of cannons and the like. It would be mostly ranged and boarding battles, I think.
This may be better suited in a Modding forum, as the current map does not make sense for ships to be implemented.
However, support for ships would allow for modders to extend the native map with islands and perhaps another large landmass.
A few ways it could be done might be to allow for party control swapping. Ie, when you interact with the ship, it turns your party invisible, and invulnerable. At which point, the ship either fallows your party, or you directly control the ship. Either way, the ship icon will move around on the water. A simpler solution would just be to change the party icon to a ship while on water terrain.
You could either access a ship from a port town, which would basically be a menu option that teleports your party to an xy location on the map, likely just beyond an invisible pathing barrier along the landmasses shore, to a location where you can walk on water, with the ship icon for the players party of course.
All that would be needed for this to work would be the ability to teleport the party, and change the parties icon, I should think. In fact.. .it might be possible to do this in a mod. Of course... it would be another story entirely to allow the AI to make use of waterways. About the best you could hope for would be a looter type wandering ship... No cross sea lord movement.
I suppose then, your request is to allow for support for the AI to send troops to locations that are otherwise unreachable? For example.. .A water terrain behind a shores pathing border? In which case, it would be map dependent.. .Likely using either the port town as the destination for any locations beyond their landmass, then party teleporting them to the sea, where they move to the next waypoint, attacking hostiles as normal.
For example... To get to Village on Island A...
Lord travels to Sargoth(port Town), lord is moved to sea(instant move), lord moves to invisable island party waypoint by the target island, lord is moved to island port town(instant travel), lord moves to village.
Sea based caravans would likily just be restricted to port towns... keeping the landbased trade routes separate.
Again, this is very academic, considering the game does not come with a map where ships would be useful. Still... a support structure for lords would be most welcome if such does not already exist.