in my opinion Water mills were (still are) not worth the effort, even at max efficiency they can only output .25eu/t? and even when making those Water Towers, that was way to much effort for how simple a device they should have been. so here is my recommendation.
make then output a max of 2.6Eu/t (.1EU/t per block of water touching it), and a bucket equal 5,000 (1/4 as effective as lava), also allow a Pump to pump water into the watermills "forcefully" creating a slight Over-Unity System. going through all this work and also building a pump to feed these would cause them to produce slightly more energy then Solar Panels but requires additional items such as the Pump and requires alot more set-up then "place Solar Panel, attach wire, recieve power"