What about more details?
I mean the idea to move water is too common and devs probably were thinking about it :)
For example: my variant is to give player limited ponds instead of building them. He can upgrade it like a house: more beautifull every time, less trash catching, choosing fish categories and finally a pond with section for fish farm and products. Pluses:
1 Current fish ponds can give everything, making hand fishing useless in late game.
2 Devs can buff riverland farm with many usefull ponds :)