[lou1306]

Sure, it is not necessarily the pool depth. But that is almost surely the one factor that deviates the most from the "average". Paris is 50-100m above sea level which is pretty unremarkable gravity- and pressure- wise. Its water is unlikely to be macroscopically different from that of other French or Western European cities.

I am aware the above may be proportionality bias, but at the same time there is some kind of "reverse proportionality bias" at play here: the assumption that since the effect of a shallow pool are too small, they can't significantly affect the athletes. Human behaviours are very nonlinear, and even very small sensory inputs may very well "throw off" an experienced swimmer.