I think you should have to craft them, but it would be cool if some of the craftable items in game actually allowed you to collect the resources for building something larger. I think it should need the "ownership" character inside and would be treated somewhat like a fief and somewhat like a person moving on the map. The person who "owns' the ship, would need to enter the ship, and then they move around the water the same way they would move their character on land. You would be able to transfer ownership over the ship.
And I agree with the battles, to keep it simple I'd say no naval battles, just have a few different types of "ship" maps where the ships are "tied" together and you can board the other ship. Having siege equipment for boarding enemy ships and defending your ships would be pretty crucial. You could also make it like current fiefs where you can construct things on the ship to make it more useful.
I'd also suggest at least 2, maybe more types of ships. Fast/mobile ships that can carry a decent amount of troops, but not a lot of cargo. Larger caravan ships that can carry lots of troops and goods. I think the larger ships would be slower, but allow you to carry more troops, so pirates would need to have better tactics and equipment to really take over a large ship.
I think there's a lot of potential, but the map would need to either be changed, or you'd need to make it so that calradia is more of an island with water on all sides. This is just my opinion of course, I'm sure brainstorming would be good if this was ever implemented.
