Yep that's what happened with a lady we know and a neighbor the male ducks hurt the hens. And yes very messy and wet not good for the chickens. Buuut if you can't build another enclosure try to only get female ducks.Usually without issue but a male duck with female chickens can go bad.
My only real problem was ducks make a muddy mess and that's not ideal for chickens. Mine had enough room to keep out of the mud though so not too big of a deal.
Chickens could also drown in the ducks pool but that's more concern with younger juvenile chickens.