A few points:
Tsunami originally is a Japanese word.
Tsu = Harbour
Nami = Wave
The waves travel at a very high speed (around 890 Kmph) in the deep sea region.
The speed is a decreasing function as one move towards shore.
However, they are not noticed even if one is present in a large ship in the deep sea. Why? The height of the wave is just a few inches in the deep sea. It reaches the shore rapidly (note the magnitude of the speed). And when it does, the height of the wave may become astronomically high.