10 Powers Comic Book Characters Only Used ONCE
9. Harley Quinn - Breathe Underwater
In the 1999 story, Batman: Harley Quinn, Poison Ivy injected Harley with a plant-based serum which endowed her superstrength, speed, agility, and a minor healing factor.
This serum also gave Harley a lesser-known power; the ability to breathe underwater. In Harley Quinn #30, a mad man called Dane kidnapped Harley under the impression she murdered his father. She tried to explain to him that his father took his own life but Dane refuses to listen and shoved her into a pool of water to drown her. Despite being dunked underwater for several minutes, Harley was very much alive. She explained to Dane that the serum she was injected with was made from Portuguese aquavilla vines, which can survive underwater indefinitely.
Harley hasn't showcased this power since so it's safe to assume this ability was decanonized when DC Comics was rebooted under the New 52 banner in 2011.
It's also worth mentioning that Harley Quinn actress, Margot Robbie, can also hold her breath for an extended period of time. For the film, Suicide Squad, Robbie trained to hold her breath for a whopping five minutes for her underwater scene.