Hi Elaine: We're not sure about this behavior because males will usually not attack their mates. When you mention that he looks like he might be trying to drown her, leads us to believe that he may be trying to mate with her. Is he nipping her or swinging his wings at her? If so, this may be aggressive behavior. However, if he is overly aggressive he probably would have killed her if it had been an attack. Knowing that everything seems to be back to normal might indicate a small family spat. If he does start exhibiting overly aggressive behavior, you may need to permanently separate the pair so she nor the cygnets are harmed. The Regal Swan