Swimming in the Canal Du Midi is not allowed. Similarly you are not allowed to swim in any other canals maintained by the VNF in France. These include the Canal de la Robine and the Canal de la Jonction as well. Swimming in the Canal Du Midi is dangerous. The danger is due to the heavy boat traffic. Furthermore, most of these boats are sailed by inexperienced crews, i.e. tourists. The tourist boats on the Canal Du Midi (Le Boat, Hapimag, Nicols, etc) are boats that you do not need a permit for to rent and sail. Accidents with these boats happen a lot, but are not published. Boats capsizing in the locks, boats getting stuck on dams, boats simply floating away without a crew, etc. This really shows the inexperience of these tourist boat crews. So the odds of these crews to see a little head sticking out of the water ahead of their boat is very low.
Swimming in the Canal Du Midi: diving from bridges
Diving from bridges into the Canal Du Midi is also not allowed. The water is not clear and obstacles in the water are invisible. These obstacles include metal bars, rocks, or tree stumps.
Swimming in the Canal Du Midi: dangerous currents
The flow of water around the locks is also very dangerous. Many locks have channels that bypass the locks. These channels were used back iin the day to power mills. Another use for these channels is to provide water for locks downstream. For example, the Nine Locks in Beziers require a lot of water to function. To feed water to these locks in Beziers, a by-pass channel is located at every lock between Marseillette and Argens. These by pass channels are very dangerous as they can suck you in with ease.
Swimming in the Canal Du Midi: Leptospirosis
The Canal Du Midi is also home to rats and coypu, also known as the nutria. The latter are large, herbivorous, semiaquatic rodents. They are about the size of a beaver, but they do not have the flat tail like a beaver. They appear rather docile and not scared of humans. The problem is that they also poop and pee in the Canal Du Midi. If you have a cut somewhere on your body, and fecal matter from an infected rat enters you run a small chance of getting a bacterial infection called Leptospirosis. But the chance of this happening is about 1 in a million, so not even worth this paragraph of text.
Baignade Interdite = Swimming prohibited.
This sign in Homps actually gives you the code number of the law that prohibits swimming in the Canal Du Midi. The Decret du 06/02/1932 = Art 59-4 (link) prohibits swimming in public canals.
Baignade Interdite = Swimming Prohibited
Baignade et Plongeons Interdits = Swimming and diving prohibited.