Not sure if this is intended behaviour, or something you didn't think about, but watermills hitboxes make it possible to completely block small rivers.
I've tested it this way:
They get so close together that you can't go through even with a rowboat or by swimming.
Yeah, please just let ships clip through them. The watermills themselves are cool, but this is silly. Especially since you can't just destroy them by hand when someone pulls this shit.
Unless we are meant to be able to use them as water fortifications and should start designing around this new paradigm.... island fortress hearth vault when??