You did not understand the concept of "realistic" watermills. Such a watermill would work with a huge tank/lake being filled by RAIN. And this would be even more laggier than solar pannels (assuming you'd need more to generate the same power), if you wanna make special bloc to calculate the tank/lake size, speed of refilling etc.
Just because MC water is INFINITE, and water stills also seems to fall continuously, which makes any kind of generator based on them an exploit.
All others kinds of "Advanced" Watermills are pretty much exploits-generator that you can use anywhere. In this case, why not just adding an expensive generator that generate EU without ANY requirements ? That sounds stupid, but it's basically the wame thing.