You deliberately threw your son into the water to see what happens because a random woman told you so? Schocking. Did you ever consider how that experiment would feel to your kid?
However, reading up on that now, the fun thing seems to be that no one can reliably tell when that reflex stops. Could be a few months, could be just a few weeks, so the recommendation is to just not do it. TIL. In any case, throwing a child in the water should just not be done. Period.
If it's not OK to hit an adult with a stick, it's not OK to hit a child with a stick! And it boggles my mind that this requires explanation.