The law is to prevent teenage boys and men from basically wearing one pair of board shorts for the entire summer and sleeping, walking, swimming in them. It's a hygiene issue. Generally the law applies in every public pool but it is strongly enforced only in public and municipal pools ie council owned ones and possibly at municipal camping sites. Private sites will have the rule but enforcement will vary.
After many years swimming here dh actually prefers his Speedos, though he goes for the boxer cut ones now!
It has been a source of consternation on mumsnet before ! Got all organised last year, but didn't seem to be an issue ( thankfully).. it was enough of a problem persuading DH to go to France by saying that I spoke fluent french ( I don't!) let alone having to force him into some budgie huggers that I had secreted in the case!
Last year we had a fab holiday at Camping Bel at La Tranche Sur Mer. They advised that swim shorts could not be worn ( ie should wear speedos... obv not nude ), but actually it was all very laid back and no one seemed to bother...
This year we are going top Les Ecureuils, and the website says no swim shorts.... Have any MNetters been there this year who can advice if this is really the case ( before I go to Decathlon to stock up for DH and DS!)