Catherine Breed was in the Pacific Ocean near San Diego, about three days away from becoming the first person ever to swim the entire California coastline, when the captain of her support boat spotted a mako shark roughly 100 yards away.

Mako sharks are considered the fastest sharks in the ocean. But what would have been a terrifying moment for most people hardly fazed Breed, an endurance athlete who had already encountered hundreds of other sea creatures, powerful storms and unpredictable currents since her ocean odyssey began at the California-Oregon border on July 1.

Breed called the shark sightings “a small part of the journey.” Still, her crew took no chances. “I think within like 10 seconds of me seeing the shark, she was on the boat,” said the captain, Maisie Bristow.

Breed completed the roughly 890-mile voyage on Monday morning, stopping about 2 miles offshore from Tijuana at the California-Mexico border.…