Perhaps I should have done a little more homework before posing this question. A little more sniffing around led me to the surprising fact that there's an iPhone app called Paris Piscines, which shows pools, dimensions, hours and price. Less clear how lap swim works, but I guess that's something for onsite investigation. Looks like there's a 25m pool nearby.
So now, I'll adjust my question: anybody have any insights culturally speaking about pools in France? No-no's? (Ou non-nons?)