At sea a tsunami is not a big deal
At see a tsunami is a very long, very fast wave. Think 100km long, moving at 700 km/hour. This is long enough that it is governed by the physics of shallow waves even in deep ocean.
So there is a lot of energy, but it is distributed over the entire depth of the ocean and won't really be noticed by ships on the surface.
As the wave approaches land, the energy doesn't diminish, but it compresses. What had been a little wave when that energy is distributed over a column of water 5 km deep is not so little when it is distributed over 100m. As it approaches shore the wave becomes slower and higher (still the same energy, just distributed differently).
When it hits land, the first visible effect is that all of the water disappears (you're seeing the wave attempt to deliver a trough that is 10-30m deep, but there isn't that much water to go away), and then it comes back - fast. Because the land doesn't participate in the fluid motion properly, the water surges up on land and then doesn't drain out when the next one hits. And this continues for as many waves as there are in the wave train. (I think that this one had 2.)
Hopefully you never need to know this, but if you take away anything from this explanation, it should be that if you're standing near the ocean and the water disappears, run!!! If they'd known that, many of the victims of this tidal wave would have survived. Sure it hit a lot of coast. But anywhere that it hit, if you were a few blocks inland when the wave came, you survived.
Cheers,
Ben
