TOBA, Mie Prefecture—For more than 20 years, Yoshiji Nakamura has often single-handedly cleared his island beach of garbage and debris, only to see more waste drift in and ruin his cleanup effort.

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In the cool waters off Japan’s Ise-Shima National Park, Aiko Ohno, a seasoned ama diver, glides effortlessly through the sea. Using only goggles, fins, a weight belt, and a mesh net called ...