After reading to this point, I must interject that you are in my opinion a very foolish person, to compare a tsunami to your possible experience in Hawaiian waves. This is like saying since I have survived a bite from a poodle, I know the trick to surviving a mauling by 100 starving hyenas.
The period of a tsunami is measured in miles, there is no "back side" to the wave. This is like saying you would dive under an approaching flash flood in a canyon, and hold your breath and pop up.
Sorry fella, but strong swimmers can swim 1.5 to 2 MPH. Against a 30 knot tidal bore, tsunami or flash flood, this is nothing. You are a ragdoll in a torrent, carried where it will carry you. Up, down, around. You are at the mercy of the wave.
As far as tossing off anecdotal "my big wave stories" to establish bona fides, I've been in 50 foot waves. Hundreds of them! But they were harmless, post-storm mid ocean waves with 100s of yards between. Just rollers, enormous swells. I've also been in 15 foot waves in the ocean on sailboats, with square vertical faces which were totally terrifying. I know from waves.
A tsunami is not a "wave" in any sense you are spouting off nonsense about.
People did survive in boats offshore and underwater when the tsunami hit. You may not know as much as you think.
You wrote...
"This is like saying since I have survived a bite from a poodle, I know the trick to surviving a mauling by 100 starving hyenas."
Excellent comparison.