While the Cult of the Abyss, the Deep Ones, and the Mi-Go battle for the fate of the ship, the RMS Olympic is still a luxury liner filled with hundreds of normal people with normal, messy lives.
Here are three distinct side-plots the GM can use to distract the investigators. These mysteries look like "Mythos activity" on the surface, but are actually completely unrelated, making them perfect red herrings.
The Setup: A massive ruby necklace (or a large sum of cash) has been stolen from the First-Class safes right as the fog rolled in. The investigators might find a cracked safe or an injured passenger and assume cultists are gathering funds or ritual materials.
The Reality: There is a highly skilled jewel thief operating on the ship, using the chaos of the "storm" to crack safes.
The Thief: Evelyn Moreau, a first-class maid whose innocent, helpful demeanor is a flawless cover. She uses her master keys and knowledge of passenger routines to steal from the wealthy. She is just trying to save enough money to open a dress shop in Southampton.
The Victim/Rival: Don Rafael Mendoza y Valverde, a desperate, disinherited Spanish noble fleeing massive debts. If Evelyn stole his last stake, he might be stalking the halls with a revolver, looking like a crazed cultist hunting for blood.
Why it works: Evelyn sneaking through the dark halls, picking locks, looks exactly like cult sabotage. If caught, she has no idea what a "Star-Seed" or "Cthulhu" is—she just has a pocket full of stolen diamonds and is terrified of the monsters.
The Setup: The investigators hear rumors of a "monster" or a "fish-man" hiding in the First-Class Card Room. They find a man constantly sweating, shedding skin, and smelling of low-tide. The investigators will immediately assume he's the mastermind behind the Deep One boarding party.
The Reality: Monsieur Henri Duval is just a man having the worst vacation of his life. He received a "blessing" from the Esoteric Order years ago, never thinking much of it. Now, the overwhelming proximity to the Star-Seed of Dagon and Point Nemo has hyper-accelerated his transformation into a Deep One hybrid.
The Twist: Henri is incredibly wealthy, completely cowardly, and deeply ashamed of his mutating body. He is hiding in the Card Room, pressing a damp cloth to his gills, terrified that society will see him.
The Interaction: He doesn't know Ybryxu. He doesn't know Martin Bell's plan. He will actually pay the investigators a fortune to hide him from the other Deep Ones, because he considers himself a high-class Frenchman, not a deep-sea fish! He's a monster, but he's a snobbish, entirely harmless monster.
The Setup: Investigators find Lady Arabella Worthington barricaded in the First-Class Library Lounge. She is surrounded by ancient, esoteric sea charts and is babbling about her dead husband's ghost whispering to her from the corners of the room. She seems like a textbook cultist who has read too many forbidden tomes.
The Reality: The sea charts she inherited from her late husband actually belong to the Tindalos sect—geometry that maps the angles of time rather than the ocean.
The Twist: Lady Arabella isn't a cultist; she's an accidental victim. By studying these non-Euclidean maps during the reality-warping energy of the ship's ritual, she has accidentally drawn the attention of Sir Zachariah Lowe (The Hound of Tindalos).
The Danger: The "ghostly whispers" she hears are actually the Hound hunting her back through time. She is a massive liability. If the investigators stay with her too long in angular rooms, the Hound will manifest through the corners to take her, treating the investigators as collateral damage. She knows nothing of Cthulhu or the Deep Ones—her threat is entirely temporal.
Don't introduce these organically as "quests." Let the investigators stumble into them while hunting for the real threat. If they are tracking Martin Bell, have them bump into Evelyn Moreau picking a lock. Let them waste 30 minutes interrogating Henri Duval, only to realize he's just a terrified, mutating man with no useful information. It makes the ship feel incredibly real and deeply unpredictable.