Actually, what happened in "The Poseidon" and "The Poseidon Adventure" (the original movie) is impossible. A tidal wave, or more correctly, a tsunami (which is what supposedly hit the ship) only crests in shallow water. When the wave is at sea it appears as an elongated lump of water with a very shallow amplitude (or wave height) only a few feet high and a wavelength of a mile or more.
A cruise ship would likely not even feel the wave pass beneath it while in open water. The wave begins to crest as it approaches the coast, all the energy of the wavelength being trasfered to the amplitude of the wave, thusly creating the wall of water.
Beyond that, most tsunami never reach an amplitude any where near what was depicted in the movies. The tsunami with the greatest height ever recorded was under 30 feet, and most are 10 feet or less, so even if a wave managed to crest in the middle of open waters, it would hardly have enough force to severely rock a huge cruise ship, much less topple it.
I have a background in the US Navy, but the information is easy to find online.
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