Got home yesterday to find one of my two female Khakis doing the backstroke in the pond (40 gal stock tank). She was 1.5 years and healthy other than soft shelled eggs. No signs of attack, the other duck and three chickens are unharmed. The pond is secured inside the run. I think they got too rough playing or attempting to mate. They take turns being the dominant one and especially like holding each other’s head under the water . RIP Red Duck.
i'm sorry you lost your duck. i'm hoping my thoughts don't come off as uncaring, but the the "?" after my duck drowned, seems like you're looking for an answer. it seems unlikely to me one duck could drown another duck. not a duck expert and never owned ducks but i've seen some pretty rough duck mating on farm ponds. i've also caught a duck in a gill at 30 feet and that sucker was under at least 8 mins and it lived. maybe some duck people can comment , i'm guessing there was another contributing factor.