A Hopeless Swimmer's Counterplan (カナヅチ対策, Kanadzuchi taisaku) is an extra chapter included with Volume 15.
While at the beach, Saitama asks Genos if he could swim. Genos explains that Dr. Kuseno has prepared some parts for him to be able to swim and left to prepare them. While relaxing in the ocean, Saitama wonders where Genos is and calls out his name. Suddenly a telescope lift Saitama, revealing Genos as a submarine with Saitama yelling at him to get a hold of himself.