John Denver's "I'm Sorry" was the singer/songwriter's across the board chart-topper. It was also Denver's biggest double-sided single with its flip side, "Calypso," gaining large-scale chart success. The latter was inspired during Denver's visit to ocean explorer Jacques Cousteau's ship, the Calypso. An environmentalist, he joined the board of Cousteau's Cousteau Society and was a member of the World Wildlife Fund, Friends of the Earth, and the Wilderness Society. Produced by Milt Okun, the million-selling "I'm Sorry" went to number one pop and number one adult contemporary for two weeks in late summer 1975. During the fall, "Calypso" held the number two pop spot for four weeks. His Windsong album went gold, parking at number one pop for two weeks.