What would it be? Feel free to be creative.
Well then I wanna be Aqua..woman!Whatever that guy from X-Men has. You know, the one with the metal claws and self-healing power. I don't want the claws, just the healing power. Maybe.
If you read Aquaman comics you'd know confidence is a secondary effect to breathing underwater. You clearly see this in Aquaman #32, when he "fights" the rabid Octopuss.