Canals.
I added 2 links for those who are not aware of the way the canal gates work.
This would be a great add to the game.

https://www.youtube.com/watch?v=B1gIm1oU9Oo

This second link illustrates an even more amazing system, I am in awe!

https://www.youtube.com/watch?v=_tBH9SE-Kw8

Having canal locks would greatly improve the game and create a new system of goods transportation between map regions.
Let me know what you think.
Do canals and canal locks are in use outside Europe?

PS If you are interested how the Falkirk Wheel (second link) works, this is a good explaination:
https://www.youtube.com/watch?v=k27gOmrDgpo
It really is an enginering marvel, apart from looking beautiful.

As cool as it sounds, without there being any transport over water in the game it would be a waste of development time to implement it.

Well, we could transport goods on waterways, especially beaver-made canals. A good canal net is the best way to transport timber, planks, metal etc etc.