Although it may appear that a large number of triathletes are swimming in the Seine, this was not the case. Those scenes were filmed at an open-air water tank in Alicante, Spain. (Famously, the Seine has been deemed too polluted to swim in, something which the French authorities have spent upwards of a billion euros to rectify.)
Lilith, the shark "star" of the movie, reproduces and has baby sharks asexually, without a mate. This part of the movie is not science fiction as females from at least 2 species of sharks have had pups without mating.
Xavier Gens found himself turned down by every French production company he approached. Netflix came to his rescue.
The film landed on Netflix a month before the 2024 Paris Olympics (which is referenced in the film).
The bulk of the underwater scenes were shot at an indoor water stage in Belgium.