There's a plethora of Star Trek erotic fan fics out there where Kirk and Spock have gills. I'm serious. It's straight-up smut, except the fact that the two of them can breathe underwater for some reason I can't possibly explain -– until perhaps now. This 1973 episode of the Star Trek cartoon is, I believe, the first time Kirk and Spock palled around underwater; I don't find it particularly arousing, but maybe you'll find something I missed. Grab your swimtrunks and your floaties for Star Trek: The Animated Series' "The Ambergris Element."
The Enterprise is orbiting Argo, a planet mostly covered in pea soup-green water. Argo was once a "land planet" which had some kind of earthquake problem which led to the rise of the oceans; apparently there's another unnamed Federation planet that's about to have a similar problem, so Kirk and crew are going to study what happened on Argo to see if they can help the new planet.
Kirk, Spock, Bones and a redshirt head down to explore the planet's oceans in a special aqua-shuttle, when they are almost immediately attacked by a "sur-snake" which looks like a giant, angry shrimp. Kirk and Spock are thrown from the shuttle and lost, while Scotty and the still-living redshirt (weird, right?) stay with the wreckage and are picked up. Amount of studying done: 20 seconds, tops.
Five days later, Kirk and Spock are still missing. The Enterprise continues to search by having Bones stand on the aqua-shuttle with a pair of binoculars while it sails around. Somehow, against all odds, Bones spots the duo, lying down unconscious on a small outcropping of rock.
Bones notes that the two are in perfect health with the tiny exception that their lungs don't work. Bones does not seem particularly concerned by this, or the fact that Kirk now has webbed hands. When Kirk — and his non-functioning lungs — wakes up and complains he can't breathe, possibly because his lungs don't work, Bones reaches the conclusion that they "can't live in the air any more." This stunning development does not invoke any kind of facial expression on anybody.
Two days later, Kirk and Spock are chilling in a tiny room full of water. They are still wearing their Starfleet uniforms, complete with socks and shoes. Bones' tests reveals a mysterious substance in their bloodstream which has changed their metabolism to give them gills, webbed extremities, and a film over their eyes like a fish's second eyelid which the animators don't bother to convey in any manner.
Kirk and Spock don't remember anything from the past five days, Bones assures them that they had to have been injected with the mysterious substance, because science. Unfortunately, science cannot explain what the substance is, or how to reverse it. Bones seems perfectly content to just let Kirk and Spock have gills for the rest of their lives, but Kirk suggests he and Spock take a swim back on Argo to see if they can find what — and who –- transformed them.
What do you know, there's intelligent life on Argo! They're the Aquans, green-skinned fish-people who hate the hell out of all "air-breathers," especially "air-breather spies" because of some "ancient records." The fact that Kirk and Spock meet them several leagues below sea level and are obviously breathing water does not seem to affect their status as "air-breathers." The fact that there are no air-breathing creatures on their entire goddamned planet also makes me question why the Aquans are expecting spies, and if there were any, what the hell they'd be spying for.
It turns out the Aquans found Kirk and Spock and "saved" them by by injecting them with the mutagen that the Aquans used to mutate themselves back when Argo first got submerged. The Aquans elders are outraged at this act of kindness towards the hated air-breathers, although no one seems to realize that these guys 1) gave Kirk and Spock gills and then 2) immediately put them on dry land. That's not kindness, that's a dick move! They obviously could have just taken Kirk and Spock to the rock outcropping, but no, they had to destroy their lungs and add gills first.
Anyways, thanks to those crazy ancient Aquan records, mutating people into water-breathers is fine, but turning water-breathers into air-breathers is forbidden. Kirk and Spock are trying to explain that they come in peace when an Aquan lady arrives and says she saw three more air-breathers –- Scotty and the rest of the landing team that had brought Kirk and Spock down to Argo -– which all the Aquans immediately decide is some kind of terrifying five-man air-breather invasion. In punishment, the Aquans take Kirk and Spock back up to the surface and leave them on another rock outcropping. So. In order to be kind, the Aquans give Kirk and Spock gills and left them on a rock to die of suffocation. And later, in punishment, the Aquans also left them on a rock to die.
Luckily, an Aquan named Rila, who feels that perhaps leaving people on rocks to die might not be the cure-all her people believe, rescues Kirk and Spock with the help of Scotty, who is glowing and can breathe underwater thanks to the completely unexplained power of a "life-support belt." Scotty warns them that a major underwater quake is hitting in about two hours.
Rila says there are legends of these quakes –- then explains that when the quakes originally happened the Aquans mutated themselves to live underwater. This begs the questions: Why is the big earthquake a legend if they obviously remember it happening? And why do the Aquans hate air-breathers if they used to be air-breathers? Rila goes on to explain that after a few generations the mutations became hereditary, which seems like it would have been exceedingly problematic for those first few generations when it wasn't hereditary and they had to mutate all the Aquan babies the second they escaped the womb.
Rila goes on to explain the cure can be found in the ruins of the ancient city the Aquans came from, although this is totally against those ancient records. Kirk stresses the importance of the task, saying that the Aquans ancient records could help that other unnamed planet that about to have their own water-quakes, although 1) the Aquans clearly weren't able to help themselves when their water-quake hit and 2) how a formula that turns gill-people into lung-people is supposed to help the surface-dwellers of the other planet is nonsense. Basically, Kirk is bullshitting the nice fish lady.
Kirk and Spock find the city, grabs some scrolls that just happen to have been kept in airtight containers and have a helpful winged merman on the label, the universal symbol for CURE FOR FISHMAN-ISM. Kirk and Spock swim away from the city when they are immediately attacked by one of the giant shrimp monsters for about 10 seconds until a giant stone wall connected to nothing falls on it.
On the Enterprise, Bones reads the scrolls and learns the key ingredient in the cure is the venom of the giant shrimp monster. And they just had one! So Kirk and Spock go back down to Argo, where they are helpfully immediately attacked by another giant shrimp monster which is also immediately dispatched by another a giant stone wall connected to nothing falling on it. Kirk uses attaches some kind of device to the monster's fang to suck out the venom –- in what maybe be the most situation-specific tool ever seen on any episode of any Star Trek ever — and leaves.
Bones makes the cure, after a lot of hemming and hawing about how he doesn't know the correct dose, and Kirk briefly changes colors and gets scaly but of course it all works in the end. The weird aside is that Bones mentions he's been testing the cure on fish while trying to find the proper dosage, which means somewhere on the Enterprise is a pile of fish with lungs who are very, very confused.
With Kirk and Spock cured, the Aquans –- who no longer mind air-breathers, the fact that Kirk and Spock broke into the forbidden city, or anything else for some reason –- visit the Enterprise just in time to see one of the Enterprise's phasers little-known uses –- shooting planets to move earthquakes. Yes. That happens. The ship shoots the planet and earthquake moves somewhere nice. Spock points out that the other planet will be safe now that they've learned their totally preposterously planet-shooting technique.
The relocated quake even unsubmerges the ancient forbidden city, which delights the Aquans even though this has technically made it harder for the Aquans to get to. The Aquans tell Kirk that the young Aquans are going to live on the surface now, although the old Aquans are too stuck in their surface-hating ways to give it a chance. When Kirk asks them to make sure they keep in touch with each other, Ryla says they'll make a new ancient record forbidding it -– and not ignore it this time! Even though ignoring most of ancient records has worked out for everybody so far! Yay for arbitrary laws with severe punishments! Whee!
So what did we learn?
• Starfleet regulation demand officers wear socks and shoes, even underwater.
• Dropping people off on rocks they can't breathe on is appropriate for all situations, apparently.
• Phasers have a "move earthquake" setting.
• There are a lot of giant stone walls in the ocean, and they hate giant shrimp.
• There's nothing worth doing that isn't worth making a law forbidding people from not doing it.
• This episode is desperately lacking Bones yelling, Dammit, Jim! I'm a doctor, not an ichthyologist!"
• Seriously, there's a lot of Kirk/Spock mermaid slash fic. Way more than you'd expect.