Superman as a merman
(...and his mermaid girlfriends)
Superman and his friends are no strangers to transformation. As seen in For Whom the Gods Destroy, Superman was turned into a centaur. In Lois Lane #92. Lois becomes a centaur. Her transformation came as a result of her somehow knowing Biron, who himself was transformed from a centaur into Superhorse. And of course there's Jimmy Olsen who's friendship with Superman has resulted in his transformation into everything from a yellow blob to Turtle boy.
In the "Golden / Silver Age" of comics, Superman found himself attracted to Lori Lamaris a mermaid from Atlantis (Of course). Lois Lane, in spite of constantly trying to "out" Superman's secret identity of Clark Kent, was in love with him. In several issues of both the Lois Lane and Superman comics, Lois was driven to despair as Superman married Lori. In one issue, Superman himself took on merman form, curiously retaining his shirt belt and cape.
So, without further history, images from comics featuring Superman and / or Lois' interaction or transformation into merfolk follow: