The water is "so clean" that you have mandatory showers before going in... But it's France, so how would you live without pointless regulations?
But not so much for hygiene standards (the chlorine takes care of that), but more for maintenance issues.
Rinsing off before going in reduces the organic load in the pool, which reduces the amount of chlorine consumed, which reduces the amount of chemicals needed to maintain clean water (not to mention reduces chloramine levels which can irritate swimmers lungs).
It's absolutely disgusting.
> Y a-t-il des douches ? > > Oui ! Chaque site de baignade propose des douches – obligatoires avant d’aller se baigner –, un accès à des toilettes, un poste de secours ainsi que des espaces pour s’asseoir et profiter du soleil.
https://www.paris.fr/pages/baignade-en-seine-toutes-les-ques...