What’s large? I’m not sure the creeks and swimming holes I frequented in my youth would qualify.

Also, the post isn’t about swimmable water—it’s a cautionary tale about how little it takes to drown. I can confirm that as one winter my neighbor drown in a shallow mud puddle off his back porch. He had a non-fatal heart attack and fell into it unconscious.

Large is exactly 20910 liters or more per square foot, absolute cutoff, and if you go even one under it's no longer large.