In the early 16th Century, the brilliant Leonardo da Vinci was in Venice. That wealthy city was in dire straits, as its navy had been badly defeated by the Ottoman Empire. There was serious reason to think that the city itself might fall to the Turks. The Ventian government needed solutions and there was no better mind to develop them than Leonardo's.
Among the many ideas he sketched in his notebooks was a complete design for an underwater diving apparatus, a reconstruction of which is pictured above. Cara Giaimo describes the suit at Atlas Obscura:
The most complete plans show a leather suit and facemask, with goggles and an inflatable wineskin to enable sinking and floating. Two hollow breathing tubes, made of cane and reinforced with steel rings, lead from the diver's mouth up to the surface of the water—some incarnations show them attached to a floating disc, while others have them leading to a pocket of air trapped by a diving bell. There is even a special pee pouch for the diver, ensuring he can stay down there regardless of whether nature calls.
Some historians think this suit was part of an elaborate plan to attack the Ottoman ships from below, in order to sink them or release prisoners. Others, including McCurdy, say it more likely dates back further, to da Vinci's time in Milan, in which case he may have intended it to attack Venice instead. (It was a time of tumultuous alliances.)