Every generation before me was taught to swim by being tossed in a pond. It's a dangerous world. Pretending it isn't just lowers the minimum required stress before anxiety takes over.
What makes it worse is that OP described it as an experiment that was primarily for his benefit, because he wanted to see what it would look like to see kid drowning. No word of wanting his kid to learn something (which still wouldn’t have been ok but at least barely makes a tiny bit of sense). That is mental, and child abuse.
A bad parent subjects their child to an intentional near-drowning, leaving the child afraid, confused and fundamentally unequipped for the danger.
This is how it is now, and this is how it was in previous generations. I do accept that bad parenting was more prevalent in the past, but then, so was leprosy.