In matter of fact, Armagan himself said that they are considering naval combat after they finish the game.
Obviously, naval combat is something else. You need to think about more stuff like if they ever implement ambushes, how's that gonna play out in the sea or how naval combat will be like etc.
But what OP asking is actually quite easy to implement.
- Go to the city
- If a city has CanSeaTravel in XML, show an option such as Travel with Sea,
- When a user clicks, list all the port cities and how long will it take to reach there and it's cost based on your army size etc.
- Once the player selects a city, don't even bother to give control to him, simply move a ship from location A to B and let world continues as if you are moving.
- Congrats, you sailed from Empire port to Aserai port.
If they add this into game, they can later add naval combat easily as well. They can even start with random events, where you can randomly encounter pirates/bandits/raiders based on a random possibility and it can simply prompt their negotiation screen. Your team still didn't figured out how to make naval combat? No problem. Remove battle yourself option like Total War did. Only use auto-resolve until you implement that battle feature.