Venice Mayor Luigi Brugnaro has stopped a swimmer in the Grand Canal.
The man had jumped into the water near the train station in his swimming trunks, as television footage showed.
This is forbidden in the lagoon city and will be punished with hefty fines.
Brugnaro was walking around town when he saw the swimmer on Thursday and addressed his behavior.
Then the police rushed over to take care of the apparently drunk man, as the Telechiara broadcaster showed.
Canal swimming is punishable by a fine of 350 euros in Venice.
The Unesco city has always fought against tourists who jump into one of the many canals from time to time and thus endanger shipping traffic, among other things.