No, it’s definitely not humane, but I disagree with most saying that it is an act of anger.
It is very common for farmers around here to collect their live traps in the morning and take them down to their creek and leave them for some time. They walk away, come back later, and dispose of the drowned animal (usually raccoon or possum). It’s not anger, it’s routine; and it’s cheaper and cleaner than a bullet.
As for it being a cat.. well, a predator is a predator. I don’t think that most people who could drown a raccoon would be much more bothered by drowning a cat. Maybe I’m wrong.
Please understand that I am NOT condoning this in any way.
Even some of my extended family members practice this method. It’s important to understand someone’s point of view before you can educate them on why it’s wrong, and teach them humane alternatives that fit into their lifestyle.