I’m from the U.S. I got this information from teen magazines in the 80s (because we don’t get much in terms of sex ed/what to expect during puberty/etc. here). I totally understood the need for not bleeding into shared swimming spaces (like pools) but the tone of it was that something awful would happen — to YOU — if you got into water while on your period. I always thought it would cause an infection or something because of the way that warning was presented.