Implement irrigation into landscaping so people can make small streams and ponds. Streams would be 1x1 shallow water tiles so people could walk over them easily. Ponds could start out empty and have to be filled or have to have a small stream supplying it with fresh water from a river or well. Players could then stock their pond with fish that would start spawning. Ponds would be useful inside the walled cities. With a source of water closer, fishing within your walls, and adding beauty to those of use who like to make our gardens look nice. Streams would also give players who don't have wells a closer sorce of water. I live near a river but there is no source of underground water to build a well on.
You could then also introduce rice that can only be planted on irrigated tiles as a new plant.
The problems I can see is do you have it filled with water to start, do they dry up, if you can make small streams how do they go under wall and barricades to feed your pond. There would have to be either a special wall section or walls can be built on stone tiled irrigated tiles. (Stone tiled ponds would be more like roman pools.) You could also implement 1x1 tiles that look like small bridges over the streams mostly for looks.
I cant help but picture a lovely stream flowing in from the river into my lands with a tiny bridge over it and a nice pond. silly huh.