Yes, I have almost drown when I was young. Water in the lungs is an incredibly painful experience while you try to fight for your life. It is something I hope never to experience again, or inflict on anyone else.
Waterboarding is inhumane because it makes the brain think it's drowning, do you honestly think that actual drowning is any better to experience?
I can honestly say falling asleep is far better and more humane. Feeding live is nature taking it's course.
I don't hit animals with my car to feed my other animals. That is an accident, not an explicit choice.
There is a set of euthanasia guidelines set up by the ARAV. I suggest you read it and talk with Veterinary medical professionals about the ethics of drowning a living animal on purpose.