Since you don't want to change watermills to behave like real water-plants in regards to producing electricity if not manned, would you mind changing the output of a manned watermill?
Actually a bucket of water nets you 1000 EU on a rate of 2 EU/t. I would suggest changing this to 1 EU/t while still giving a total of 1000 EU. This would make it easier to maintain manned watermills by hand while decreasing it's gain with means of automation.
With RP or BC+Addons installed you could use a watermill all day at 2 EU/t which outperforms solars greatly. By reducing it to 1 EU/t (or even 0,5 EU/t while quadrupling the time used to consume one bucket) it would match the other generators better.