Source: Superman's Pal, Jimmy Olsen #55 (1961) Type: Transformation
And yes, dinosaurs(ish) because it's still Dinosaur(ish) Week. But first things first: How does Jimmy Olsen become Aquaman?
Oh, just a little something we call the Silver Age...
Can't leave the seas undefended, right? Where did that technology come from and why doesn't Aquaman use it all the time? Why not try it with a trained, experienced superhero (Aqua-Flash would patrol the 7 seas much more quickly, for example)? Why a teenager photog whose swimming skills are unknown? I bet Snapper Carr was pissed to hear Jimmy had a go! Not just a go, he had his own THEME SONG!
Or at least his own theme poem. Jimmy immediately sets out to help people on the water. Things like give a pretty actress who force lands her own helicopter away from show a proper photo op by allowing her to walk to shore on the backs of giant turtles. Seems a very Jimmy thing to do. When asked to deal with an insect swarm carrying a devastating plant disease (go with it), he crosses paths with an iceberg containing a real (well, not real in the sense the species ever existed) dinosaur. The ice cracks and Jimmy makes a new friend.
Oh, inter-species romance! In the proud tradition of comic book animals who fall in love, it doesn't matter if Lizzie is a reptile, she's gonna have biewwwtiful lashes to show off her femininity. That prehistoric mascara isn't working for her though, and Jimmy is all, like, dissin' her and the other love-sick girls he's known. Playa gotta play. To deal with the insect swarm, he makes flying fish leap into the air and gobble it all up. But his relationship with them makes Lizzie all jealous, and she shows off how she can catch stuff in mid-air too!
Now he has to save that dude. His anger makes Lizzie cry. She's empathic, yo! But also, a bit of a copy-cat. So when Aqua-Jim shows her how to deal with a brush fire, you know she's going to go after that antique lighthouse the same way.
She knocks the top off, right into that boat, and Jimmy again has to save the day with his Aqua-powers. Too bad they can't actively control her for no discernible reason. Next up, a floating mine from the Dubya-Dubya-Two that needs to be harmlessly exploded before it hits a ship. Jimmy makes a gigantic ribbon fish throw a rock at it. But wouldn't you know it, there's also a bathysphere in the area (these convenient coincidences are so damn retro), which Lizzie takes for a mine (of course).
But wait! More dinosaurs including Terror of Mechagodzilla's Titanosaurus and a humongous water-breathing Dimetrodon (not a dinosaur, but a saurian precursor) who come out of nowhere, firing useless beams of energy from their eyes and pushing icebergs together, which...
...trap Lizzie once again. If Jimmy didn't do this, and Aquaman's still away, then how did this happen? That's the question the comic asks so we won't ask the real questions, like WHAT THE HECK?! Annnnnywayyyy, the real superheroes return from space just before Jimmy's powers run out and he drowns or something, and Lori Lemaris has a telepathic chat with Superman, telling him she saved Jimmy's landlubbing bacon before he messed up her whole environment with his pee. The End.