Rivers to Oceans

This has bugged me since the first day I played Civ 5.

My city is built on a river, this river leads to an ocean. Why can't I produce ships in the city, have them travel along the river, and get to the ocean?

I really think this should be implemented because it is a realistic and balancing feature for many civilizations. It could even be a civilization specialty for the Ottomans, English or Songhai.

That is all!