Being away at sea for a long time seems to take its toll on your mind. Maybe it's the unchanging landscape, maybe it's being away from your loved ones. Either way, in period pieces and even sometimes in modern ones, you can expect any characters away on sea for extended periods of time to go crazy.
Historically a Truth in Television. In addition, there's a lot of validity to the "don't drink seawater" idea, since salt water has the nasty effect of making you even more dehydrated, which can lead to delirium. And, y'know, death. That is, if the crew haven't already thrown you overboard in disgust.
- Hikari and Ken from Digimon Adventure 02 go mad (or at least have very bad FreakOuts) whenever they return to the Dark Ocean, though it's less from the usual reasons, and more because they basically stumbled into a Cosmic Horror Story.
- In the Omake episode of Tsukuyomi: Moon Phase, Seiji Mido has gone completely mad from being trapped out at sea for several weeks. Though he could be considered the Only Sane Man, as everyone else is completely blase at their house being intact and floating in the middle of the ocean (the giant cork just has to be left alone).
- The living skeleton Brook from One Piece. After coming Back from the Dead with his Devil Fruit power, Brook spent fifty years completely alone trapped in the Florian Triangle because of his ship's broken rudder leaving him stranded. He's not completely mad, but he has suffered Sanity Slippage that left him with No Social Skills and occasional hallucinations before meeting the Straw Hat Pirates.
- Sub-Mariner: The Depths is a miniseries written by Peter Milligan about a submarine crew slowly turning against their leader professor Randolph Stein because the crew insists searching for Atlantis will incur the wrath of the so-called "Sub-Mariner", which Professor Stein stubbornly refuses to believe, even as he starts to hallucinate and act irrationally.
- Watchmen: The Story Within a Story Tales of the Black Freighter is about a sailor who's marooned on a deserted island by pirates who killed everyone else aboard his ship and the increasingly desperate and insane things that he does to survive and get home to his wife and children. In the end, he make it home but almost murders his wife (after murdering a random stranger that he believed to be a pirate) only to realize that driving him to such insanity was the pirates' actual goal all along, and he voluntarily joins their crew.
- Referenced in The Road to El Dorado after Miguel, Tulio, and their horse Al Tivo have been floating for God-knows how long and then suddenly wash ashore:
Miguel: And it is! It really is the map to El Dorado! *panting with excitement*
Tulio: ...you drank the seawater, didn't you?
- The Little Mermaid (1989): Prince Eric wakes up on the beach and starts talking about being rescued by a girl, Grimsby replies, "I think you've swallowed a bit too much seawater."
- Results in a particularly delirious musical number in Muppet Treasure Island.
We've got Cabin Fever!
We've lost what sense we had!
We've got Cabin Fever!
We're all going mad!
- Tom Hanks again in Joe Versus the Volcano, after days of solitude, dehydration, and exhausting the entertainment potential in his luggage.
- An anti-Ocean Madness might be Jack's locker-induced madness at the beginning of Pirates of the Caribbean: At World's End — apparently, Hell for a pirate is an endless barren salt flat, far from any sea.
- Master and Commander: The Far Side of the World has a significant subplot concerning an officer who comes to believe he is a curse on the ship. He kills himself.
- Coffey in The Abyss quickly descends into ocean madness - High-Pressure Nervous Syndrome, to be precise.
- The end of Road to Morocco has a ship exploding. Bob Hope, adrift on a bit of wreckage, goes into a hyper-dramatic fit of madness, until cohort Bing Crosby points out they're practically at the docks of New York - Bob chides him for ruining his chance for an Oscar.
- Cabin Boy has an extended ocean madness scene that involves a tobacco spitting, foul-mouthed cupcake.
- The Rime of the Ancient Mariner is essentially about a crazy old man accosting a random guy in the street and making him listen to an extremely gruesome experience of Ocean Madness. Hopefully. Because if he was actually sane when he saw all that stuff out there...
- The fact that the Mariner might be close to a thousand years old would seem to corroborate his story, unless every single detail, including when it happened, is a hallucination.
- In China Miéville's The Scar, Hedrigall decides to leave Armada, the floating city, and spends some time alone at sea. After he is found he has been driven mad by seeing his entire city destroyed.
- Cryptonomicon has Goto somehow managing to swim all the way from open sea to New Guinea. As an Okinawan who grew up in the sea, he knows damn better than swallowing the sea water. Problem is, he's swimming with a Tokyo city mouse, who doesn't knows any better... and after swallowing the sea water for a while, he slowly starts losing it, until he collapses in the shore while laughing sardonically...
- Captain Wolf Larson, in Jack London's novel The Sea Wolf, starts off as a sadistic Nietzsche Wannabe, but degenerates into a full-blown psychopath with a death wish.
- From Nation: "Calenture...meant a special kind of madness...sailors got [it] when they'd been becalmed at sea for too long. They'd look over the side and see, instead of the ocean, cool green fields. They'd leap down into them and drown." Although, since it's First Mate Cox doing the explaining there, it's possible that he pushed them in himself.
- Terry Pratchett had already mentioned the phenomenon in Going Postal, where he uses it as an analogy for what happens to clacks operators after a while.
- Likely happened to Pi in Life of Pi, becoming most of the conflict in the story (the rest being stuck on a boat with a tiger.)
- Inverted in John Masefield's "Sea-Fever":
I must go down to the seas again, to the lonely sea and the sky,And all I ask is a tall ship and a star to steer her by,And the wheel's kick and the wind's song and the white sail's shaking,And a grey mist on the sea's face and a grey dawn breaking.
- Lampshaded by Wolf Boy in Septimus Heap, who thinks that he'd be driven to madness by the vastness of the sea.
- In Jam, the main character, Travis, having managed to barely escape a small scale apocalypse but losing several friends in the process, finds himself stuck floating on a lifeboat with his only company being a spider he brought along in a box. It doesn't take long for Travis, who already struggles with a severe case of Survivor Guilt, to crack completely and start talking to himself and even misattributing his own replies to his spider.
- Moby-Dick: By the end of the tale, almost everyone has succumbed to this to varying degrees, even Ishmael. The only possible exceptions are Starbuck, by virtue of being the Only Sane Man, and Ahab himself, who was already crazy before he even got on the boat.
- In The Painted Canoe, Zachariah recounts how a friend of his named Seahorse went mad after being stranded at sea for a long period of time (then went even more insane on discovering upon his eventual arrival home that his girlfriend had hooked up with a younger man). Zachariah himself almost succumbs to this, but forcibly averts this by pointedly thinking about the worst memory in his life, the death of his infant daughter.
- In the TV-movie of Noah's Ark, after the rain ends, the ark drifts for weeks across a sun-drenched and scientifically impossibly dead calm sea, and Noah and his sons and daughters-in-law all gradually go around the bend. However, Noah's wife, being the Mommeee, is supposed to be an infinite source of self-sacrificing nurturance, so when she starts to crack, all the other characters stare at her in shocked amazement and suddenly go sane again.
- On Lost the crew of the freighter is slowly going mad while anchored near the Island. However, the main cause is the time-twisting effect of the Island rather than just being at sea.
- One surreal bit-scene on The Young Ones featured a couple of rag-clad men lying on a ramshackle wooden raft. As they discussed whether or not they were hallucinating, their raft alternately appeared floating at sea, then lying on the floor in a cellar, then lying in the cellar of a house that was floating at sea.
- Alta Mar: Seems to afflict Captain Santiago at least once a season.
- Mastodon's "Aqua Dementia". The title is kind of a dead giveaway.
- Nautical Folklore: There are various tales of a sailor who would bring the most unattractive woman he could find along on his voyages (to serve as a cook, or whatever). When she began to look good to him, he knew he'd been at sea for too long and that it was time to head home (or at least to the nearest port).
- Sunless Sea, the Fallen London spinoff. "Go Insane, Eat Your Crew" is one of its main slogans, and indeed one of the major challenges of the game is keeping the terror of the Unterzee - the vast underground windless waveless dark monster-infested sea said to be the skull of a dead god - from driving your crew violently insane. Venture up to the safer, choppier seas of the surface and something similar applies; you and your crew are too much part of the Neath now and sunlight is harmful, dazing everyone to the point that they aren't afraid as their fellows start dying of it.
- League of Legends
Pyke: Bottom of the sea's the top of another world.
- Pyke was dragged to the very bottom of the ocean by a beast he harpooned. After coming back, he speaks of untold horrors waiting in the depths that would drive any man mad.
- Nautilus was likewise dragged to the murky depths. Awakening to find himself fused to his giant diving suit, he was forced to trudge slowly back to land. Only to find it had been so long, everyone he knew was gone and nothing of his old life was left. He now lurks in the waters, a bitter Evil Debt Collector for the goddess Nagakabouros.
- 8-Bit Theater: After the Light Warriors go down in a submersible, Black Mage goes nuts in an Apocalyptic Log detailing how the others also went mad weeks prior, along with illustrations of tentacled deep-sea creatures. Cut to the others asking him to stop narrating, and that they've only been there for less than three hours.
- In The Simpsons, Homer, Bart, Ned and Todd start to succumb to this after they get stranded out at sea in a raft.
- The Trope Namer is Futurama, despite this instance actually being a Cassandra Truth. When the crew gets stranded underwater, Fry sees a mermaid, resulting in everyone brushing it off as ocean madness. However, Fry really did see a mermaid, and no one actually goes mad.
Fry: Every time something good happens to me you say I have some kind of madness. Or I'm drunk. Or I ate too much candy.
Leela: It's ocean madness all right, the sailors call it "Aqua Dementia". The deep down crazies, the wet willies, the screaming moist...
- Xavier: Renegade Angel has a variant. (SAAAAAAAND MADNESS!)
- The Marvelous Misadventures of Flapjack: The episode "How the West was Fun". The full sequence is mildly horrifying.
- One episode subplot of The Adventures of Jimmy Neutron, Boy Genius has Carl, Sheen and Libby stranded out at sea in Jimmy's hovercraft, after it runs out of gas. Before long, the subject of cannibalism is brought up, and Sheen tries to harm Carl when he finds out that he sneaked in candy bars. Libby, who was being the rational one up to that point, wants Carl alive because he might have more food and they gang up on him.
- In American Dad!, after the hijacked ship Stan took over during his Jesus phase blows up due to Jeff not checking the temperature of the engines, Steve and Roger (with a wild wolf Roger picked up) are stranded on a boat in the middle of the ocean. Coupled with Roger's abysmal skills at survival, he is afflicted with insanity for drinking sea water and ends up skinning the wild wolf alive because it found out his "secret" whatever that was.
- One of the most infamous examples is the claim that sailors in previous centuries thought they had seen mermaids while at sea. It's generally assumed that they actually just saw manatees who feed their young in a manner that resembles a human female breastfeeding her baby. Still, you have to be pretty insane to confuse a manatee for a human female, or at least imaginative and needing tales to awe landlubbers with. That, or very very desperate. Columbus wrote in his log "I have seen the mermaids, and they are not so beautiful as tales tell."