Iza leaned down into her board, slicing through the surf at Linda Mar Beach, the cheers from spectators filling the air. Moments before running aground, she leaped off the board and gave a shake. Iza, a 5-year-old French bulldog, was one of more than a dozen canine competitors who traveled to Pacifica from as far as Brazil to participate in Saturday’s annual World Dog Surfing Championships with their humans. Dogs surfed alone and in tandem with their humans, in roiling waves that one human compared to a washing machine. Thousands of spectators filled the beach, braving fog and 55-degree air,...