it showed the guy drowning in the video sometimespink haze wrote: If anyone figured out the no. 1 reason it's titled 'Trial by Drowning,' you get a gold star.
well honestly, i know nothing about the anime, but from what i can interpret from this video, you coined it from the phrase 'trial and error.' While normally the phrase is to represent the usual learning process where errors are common, here, we have the hero's errors being more deadly. They hold more weight on him and thus cause him to drown under its pressure as he fights against it.
It's also interesting because you show a lot of imagery of life and power. All life comes from water, but you also show him drowning in it. So that which provides life can also cause death. A lot of the blood shown flows like water too.