Any tsunami video I look at barely looks like a 20 feet wave hitting shore. But wikipedia tells me there have been dozens of 100+ feet tsunamis even in the last 10 years.
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Think of an oceangoing earthquake generated tsunami as more like the world’s fastest rising tide, vs a traditional wave.
It’s a column of moving water that goes from the surface all the way down to the ocean floor.
This column travels at crazy speeds across open ocean, basically jetliner speed is the common comparison. Boats out in the open water may only experience a 50cm or 1m vertical change in the water surface, practically unnoticeable
Then as the ground under it lifts coming up towards shore, the tsunami bunches up and slows down, which will cause a raise in height above the water around it. As another commenter mentioned, the tsunami wave top “breaks” much further out than traditional wind waves coming into shore would, so by the time it actually hits the shoreline it does so as a “bore” (the white frothy wall of water and carried debris right at the front). Behind the bore, a tsunami has a far longer wave length than any other wave, which is part of what makes them so destructive. Where other waves the surface level returns to normal quickly, a tsunami wave will continue to rise like a very strong tide, behind the bore, inundating everything.
Just because a tsunami doesn’t look like the mountains of water you’d see in a movie, don’t underestimate its power. Even 10ft of water
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