Julia Child, before she was a celebrity chef, was a US intelligence officer who during World War II tried to develop a shark repellant for the Navy. It didn't work at all, and the search for an effective shark repellant continues to this day.
From Smithsonian:
Modern researchers have (mostly) ditched the chemicals and opted for a magnetic strategy. The secret to shark perception seems to lie in specialized holes on their snouts called the ampullae of Lorenzini. According to the going scientific theory, these help sharks pick up on electromagnetic fields underwater, allowing them to track prey and navigate….
In 2012, the Australian government tried to test the effectiveness of some electronic repellants by sticking them on seals. Sometimes they worked; sometimes the sharks ignored the electronic shield. In a 2008 test, a shark ate a repellant device…
