It’s not the sea, it’s lava on a Hawaiian beach.
In May 2018, the Kilauea volcano in Hawaii explosively erupted. It started throwing ash 30,000 feet into the air and spewed fountains of thick lava that reached the Pacific Ocean. It also traveled great distances from its origin, destroying Hawaii’s largest natural freshwater lake. It reached beaches too, filling Kapoho Bay, and extended new land nearly a mile into the sea.