This is the theory that at some point in human evolution we came down out of trees and became semi-aquatic, kind of like seals or something. Scientists generally consider it to be r-slurred bullshit, but I'm not convinced that it's 100% wrong. Maybe we weren't like seals... we were like beavers! 
Think about it. Humans develop the intelligence and manual dexterity to build complex systems of dams and ponds and stuff, and that's going to give them a great advantage. We know that in certain places fish and shellfish could be a windfall of food for prehistoric humans. Is it so hard to believe that this drove our evolution a bit in certain directions? Where I'm skeptical is I would really need to have good evidence that humans are that much better at wading and floating than other apes.
(Stolen from /r/classified, where there's many hundreds of good posts to rip off and put on /h/spooky.)
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Mayo surface dwelling cave ape devils probably hunted the majestic ebinoid sea-apes to extinction and brainwashed them into thinking they can't swim
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