Swimming pools have rules. Of course they do. Most people know that in France men must wear briefs, and are not allowed to wear shorts in the pool. This is for 'reasons of hygiene' (I suspect for the same reason outback motels in Australia have a notice saying you are not allowed to swim in your clothes -- too many instances of sweaty truckies stepping out of their truck and into the swimming pool and clogging the filters up with dust and other debris that can be avoided if you just change into a swimming costume first).
But this sign (pictured) caused me first to discuss with Ingrid what she thought it meant, then to look it up. APNEE is in capital letters so I wondered if it was an acronym (the French love an acronym).
We both wondered if it was connected to apnea, perhaps one of those words that in English is rather formal, but in French is a perfectly ordinary word. This turned out to be closer to the mark.
I consulted a couple of French online swimming forums. It was then clear that the sign meant: 'Holding your breath underwater is forbidden without authorisation and individual supervision of a swimming coach.' Plongée en apnée can mean free-diving or snorkelling. In practice what is forbidden at the swimming pool is sitting on the bottom of the pool holding you breath. The reason is that the lifeguard may not be able to distinguish that from someone who has drowned. Technically you are not supposed to swim lengths underwater unsupervised either, but in practice most swimming pool lifeguards tolerate that.