Dredge

A Fisherman sails to the coastal town of Greater Marrow, situated in a distant archipelago, to take up a job as the town's angler. While the job starts typically, the fisherman begins to encounter strange phenomena, especially at night: a strange mist, rocks that suddenly appear from nowhere, ghost ships, hostile wildlife, sea monsters, and he has a feeling of being watched. As he sails around the archipelago, he finds messages in bottles, featuring diary entries written by a newlywed woman named Julie, who recounts her arrival in the archipelago with her husband. She witnessed him dredging up from the sea floor a mysterious casket which he opened, releasing the supernatural fog that plagues the archipelago. A gargantuan sea monster called the Leviathan then awoke and destroyed their boat, killing Julie and stranding the husband and the old mayor on a small island. The Fisherman finds himself stalked by the Leviathan, which is the archipelago's guardian and is set on preventing the Fisherman from leaving the area.
The Fisherman meets the Collector, a man living alone in an island mansion. The Collector tasks the Fisherman with exploring the other archipelago islands and the ancient ruins they hide to recover several relics. The Collector uses the Book of the Deep to grant the Fisherman supernatural powers. Once the Fisherman collects the relics, he can hand them over to the Collector.

When the Fisherman hands over the relics, the Collector reveals that he is Julie's husband, who recovered the Book of the Deep from the casket and discovered a ritual that could resurrect Julie. He had the Fisherman gather the relics, all personal items of Julie's, because they are required for the ritual. The Fisherman takes the Collector to the part of the ocean where Julie had drowned and performs the ritual by throwing the collected relics overboard. The ritual successfully resurrects Julie but also awakens a massive Cthulhu-like eldritch beast, destroying Greater Marrow and presumably the rest of the world.
If the Fisherman finds the old mayor in one of the five camps scattered across the map, a new ending is unlocked. This involves speaking with the lighthouse keeper and discovering that the Collector is a figment of the Fisherman's own imagination, an avatar of his own repressed memories and guilt over Julie's death. Rather than follow the Collector's bidding, the Fisherman heads out to sea and throws the Book of the Deep into the ocean, resulting in him, the book, and his boat being swallowed by the Leviathan. Afterwards, the supernatural fog plaguing the archipelago disappears. Alternatively, you can proceed with the first stated ending. The only difference will be that the pronouns will become second person.

The Pale Reach

The Fisherman heads south of the archipelago to discover the Pale Reach, a frozen biome that appears and disappears every few years. He helps a nature photographer capture a photo of a ravenous creature resembling a narwhal and assembles an icebreaker ram for his boat. The Fisherman then pursues the trail of an Antarctic expedition crew that disappeared nearly 100 years ago while investigating the Pale Reach. He finds journal entries that reveal that the Captain, Navigator, and Boatswain of the crew became enthralled by the ravaged and still-living remains of an enormous eldritch monster trapped in the ice. As the crew attempted to free the creature under the promise of treasure, the First Officer led a mutiny, which ended in most of the crew dying or fleeing the Pale Reach. Four men remained and became trapped in the ice, still bound to the creature and unable to die. After finding enchanted ice axes, the Fisherman frees the men from their torment, with their deaths killing the creature as well.

The Iron Rig

The Fisherman comes across an oil rig owned by the Ironhaven Company and meets two of its chief staff, the Foreman and the Scientist. The Foreman asks the Fisherman to recover lost supply shipments so they can complete the construction of the oil rig, while the Scientist requests that the Fisherman collect samples of the local wildlife for study. The Fisherman brings back enough supplies for the Foreman to finish construction of the oil rig, and they drill into the sea floor. This causes fissures to open up on the sea floor, releasing a dark ooze and species thought to be extinct. The Scientist has the Fisherman collect samples of the new species; eating them causes the Scientist to mutate into a monster, and he escapes to the ocean. The Executive arrives and orders the Foreman to continue drilling, even though it angers the local sea monsters. Fearful of the monsters, but equally fearful of the Executive, the Foreman asks the Fisherman to sabotage the oil rig's defense system to convince the Executive to abort the drilling. With the defense system down, a tentacled sea monster attacks the oil rig, destroying the drill before being eaten by the Leviathan. The Executive flees via helicopter, and the Foreman agrees to continue providing the Fisherman with advanced equipment until he and the rest of the oil rig crew can evacuate.