In Superman’s Girlfriend, Lois Lane #12 (August 1959), Lois is visiting Hawaii to celebrate it becoming the 50th United State of America. She gets into an accident underwater, has her legs crushed and is rescued by Aquaman, firmly establishing Aquaman as part of the main DC continuity. Unfortunately Lois can only be operated by an apparently insane doctor who realizes her lungs have collapsed and her legs are crushed, so he should turn her into a mermaid. Um, yeah that happens!
After initial shock, Lois tries to adapt to underwater life and forget Superman, even imagining a life with Aquaman, but to no avail. Aquaman beseaches Superman to try and help her, which results in the first comic book meeting of these two characters.
Superman performs his magic, er, I mean studies every medical text at the Honolulu hospital using his super-speed and super-intellect, insists the doctors make him an official doctor, and operates on her restoring her legs and lungs to full functionality somehow.
Anyway, it was still cool to see these characters hanging out together, presaging the forming of the Justice League of America.