Aquagirl Lisa Morel from Adventure Comics 266 (1959) sketch card commission

The first and, to my knowledge, only appearance of Aquagirl Lisa Morel was in Adventure Comics 266 (1959). She was living life as a surface dweller, but when she dove into the ocean from her boat to rescue an imperiled Aquaman she discovered that she had all of Aquaman's powers. It turned out that as an infant she was expelled from Atlantis because she was a throwback to the Atlantean's surface dwelling ancestors. Her powers faded by the end of the story, and she resumed her happy life as the adopted daughter of a kind surface dweller. A few months later, in Adventure Comics 269 (1960), Aqualad debuted with a similar origin to Lisa Morel, and he stuck around for decades as Aquaman's sidekick and partner. In a way, sinking the first Aquagirl's career before it really started cleared the way for Mera in the early 1960's, a character less derivative of the male hero of the strip. (No offense intended, Lady Blackhawk, Batwoman, etc.)