A Watermill is a Resource Building in Bloons Monkey City. It's used to generate Power, like a Windmill, but the watermill can only be placed on rivers. It's slightly more powerful than a Windmill. It's one of the few buildings that has multiple forms, each facing the river's flowing direction.
A Level 1 Watermill facing South.
A Level 2 Watermill facing Southwest.
Three Level 1 Watermills and one Level 1 Windmill.
BMCM Level 2 Watermills
In BMCM, you can place unlimited Windmills and Watermills after city level 35, due to the fact that buildings keep using more and more power. Ninja Kiwi said this was intentional.