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 sequence of the wave hitting. Modern cruise ships are often described as "a city on the waves." Now imagine a city turning over on its side and then upside down.
A shot underwater shows corpses floating by still in evening dress.
One unfortunate passenger is sent flying out of the glass elevator back into the ballroom below. Her desperate wailing is especially unsettling.
One group of passengers were enjoying themselves in the ship's pool when they suddenly find themselves being washed away before the real wave even enters the room.
An explosion causes a kitchen hallway to fill with fire, the workers seeing it coming but unable to run before they're burned alive.
The gathered survivors in the ballroom watch in horror as the "watertight windows" slowly crack under the unrelenting pressure. Panic builds as they realize what's going to happen, the captain and the singer hugging to face the inevitable as the windows shatter to flood the room.
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.
The elevator shaft. Enough said.
Especially how Valentin dies.
The ship righting itself, now a complete 180 from the majestic vessel we first saw—broken, shattered—and finally sinking into the depths of the sea, taking any scant survivors with it.