I like this idea, travelling through the nether line can be a dull experience. My thoughts about this topic and the replies I have read:
1).Regarding King Arjen's reply, I support his idea to have one ship for each route. This could make the system more realistic by creating transfer routes to go from point A to B. So let's say I want to go from Casadia to kabranth, if you see a map you can see there is no ocean connecting both of them, so I should take a ship from Casadia to Perth, then I can use Perth's nether line to go from the east part to the western part, and there take a ship to Kabranth. by doing this we can have all realms connected without having to do the actual connection between kingdoms. This would make ports useful and bigger, since you would need a ship for each route, so the biggest port of hermertia would be something really epic, turning into the central hub of the empire. Also it would encourage co-builds, since you would a ship in each end of the route for it to be active.
a map to visualize my idea, blue is ship route. The red is not the actual nether line, just the route it should make.

2). It would make trade an actual thing: Without having to travel for 30 minutes, trading items would be much easier. Maybe inside the ship there could be a "warp station" sort of like a platfrom say 5x5 so you could warp with two merchant donkeys and trade all that stone you don't need.
3). This transportation system could be used only for interrealm travel, complemented with nether lines. So if you need to arrive to a certain county, first you arrive to the kingdom on a ship and then you use the existing nether lines.
4). we should also add an in-game value. Let's say the ship needs to have three diamond blocks, five gold blocks and ten iron blocks in their materials list. This would increase a ship's value production, making it an endeavor resource-wise. Maybe this value could be a payment before the warp: gold nuggets, the quantity increasing with the distance of the travel. This could let to the creation of an actual economy inside the server.
5). Finally, I share Haedren's thought: Maybe this could be achieved with command blocks, and as such, each ship would be inspected to see if it fits the requirements, and then the kings would put the command blocks for the warp to work.