Rule 1: No mercy! - There's no concept of swimming in lanes in this country. It's a free for all in whatever direction you like. Today, I got kicked in the boob by this old lady and she just turned and asked if I'd move? I looked over to the Sens de la Nage or Direction to swim and this woman was obviously going the wrong way!
Rule 2: Be prepared to see old men wearing speedos. *shudder*
Rule 3: Must wear swimming cap. Ok, I can appreciate this rule because chlorine is terrible for your hair. I get a kick out of the people who don't know how to put on a swim cap and you see if just sitting on top of their head with all their hair practically hanging out. So much for the swim cap rule. I did once see a bald guy get stopped and told that he had to go and put on a swim cap before entering the pool.
Rule 4: The dressing area is unisex. This doesn't bother me so much but the little boys who were runnning up and down the dressing area popping their heads under the dressing room doors did bother me just little bit.
If anyone else has experience the French public pool, please feel free to add on to this list. :)