This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Poseidon
The whole situation. There are plenty of historical records (and a couple of modern incidences such as detailed in The Perfect Storm) that suggest rogue waves like this do exist and have sunk ships. Rather like the movie, as well, there'd be few if any survivors from such a sudden and catastrophic event - though those few survivors would probably have been washed clear. If you were trapped on the ship when it turned turtle, it would probably sink long before you got to an opening.
The entire air duct scene. It's dark, it's cramped, and it's slowly being flooded with water. And if Conner hadn't managed to get the vent cover off, everyone in the duct would have been trapped, knowing they were going to die, and not being able to do anything about it as the water slowly rose and drowned them one by one.
Kurt Russell does a frighteningly good job of portraying someone who's drowning.