Marcos turns out to be an amphibian merely disguised as a human and needs to stay in water for at least 8 hours to keep his skin moist. To keep up his disguise among us, he only goes into the water at night--thus literally sleeping with the fishes. As it turns out, he's part of an advance scouting/infiltration force for a race of sapient amphibians that wish to exterminate humanity and remake the world in their image. You try to warn others of their plot, but everyone who listens to you mocks you for being a conspiracy nut.
In the end, their plot succeeds. The Earth is flooded* and most of humanity is killed; the rest are enslaved in the mines that now cover the remaining land mass, never to see the light of day again.
* Part of their plan involves warming the earth to the point where all surface ice melts, expanding the current bodies of water to a more comfortable size. To this end they have infiltrated energy and media companies to pump greenhouse gasses into the atmosphere spread global-warming denialism.
_____
I wish for alien intervention to free humans from the above scenario, but without them enslaving us for a second time.