Sunfish sailboats:
These things were everywhere ca. 1974/75/76/77. When we'd go to Hardings Beach in Chatham (which, btw, is closed this weekend due to Great White Shark sightings, 4 of them to be exact), there were always at least half a dozen of these things either in the water or on the beach. I wanted to try one, but no one in my family owned a sunfish.