Indeed, the whole scenario is a nightmare. But please dont be too upset over
the poor animals. As a pest controller I have used drowning as a humane and
quick method of extermination many times.
Although it may seem horrific at first to humans, the animals lose
consciousness very quickly, less than a few seconds, and then they die
peacefully.

There is also some science to this. It is the presence of CO2 in mammals
lungs that cause the overpowering urge to breathe. When the lungs are filled
with water, that CO2 is displaced, and so that panicky feeling of having no
air is gone. Just seconds later the lack of oxygen to the brain causes
unconciousness, and death follows moments after.

I hope that helps a little, those animals wouldnt have suffered too much.