CEREMONIAL DIP Brut.
Paris' mayor hopped into the Seine and walked away clean ... a huge relief to her 'cause tons of people said they were going to relieve themselves in the river to protest.
Mayor Anne Hidalgo jumped into the world-famous river for a quick dip to show her fellow Parisians the Seine's clean enough to host open-air swim events and opening ceremonies for the Olympics.
Check out the pics ... Hidalgo's doing her best Katie Ledecky impression as she works her way across the choppy river water in her wetsuit -- followed closely by a couple other top Paris officials.