I agree that map is to small for Planes and there is no point of have freighter ships either ..
You could build an International Airport which is connected to imaginary airports outside map… if you create a good transportation service from airport to residental/commercial zones then a lot of passengers will use it .. and a lot of tourist will come to it, which you need to transport around your land 🙂 … same is for ships. Make a transport from factories to shipyards and back ..
and also a lot of tourist would rather use slow tourist boat on some river than speed train… so you have another role for ships 🙂
This feature would be great also in the future if multiplayer is implemented, so you could trade with other players via ships and planes.
Something similar is implemented in cities xl… I don’t know for you .. but planes / ships would enrich gaming experience and give trains more stuff to do 🙂