Many Many years ago my neighbor and I found a stray kitten, only about 4 weeks old that was suffering with no hope for getting better. It could barely lift its head, was full of pus running from its eyes and nose, and its head was covered with maggots. We thought we were doing the best thing by ending its suffering. So, we got a pail and drowned it. We thought it would be quick. That kitten fought with more strength than we thought it had. I still have nightmares about it. So, no, drowning is not humane.
When it's my turn to pass the rainbow bridge I hope that kitten is not waiting, or at least understands that our intentions were honorable if not our actions.