Why are tsunamis in the oceans almost imperceptible, and as soon as they reach the coast, cause destruction?

Tsunamis (Japanese “tsu” – “bay”, “nami” – “wave”) are waves, a wave of huge sizes, the reasons for the appearance of waves: underwater earthquakes (or earthquakes with an epicenter on land, but very close to the coastline), landslides and volcanic eruptions. They can also be formed as a result of human activity (atomic explosion), strong winds or a meteorite fall. Tsunami waves spread from the center of the earthquake in all directions. In open waters, tsunamis are almost imperceptible, since the distance between the wave crests is large. When a tsunami collides with land, the distance between the crests decreases sharply and due to this, the wave can reach a height of 10 meters, but inside the bay the tsunami narrows by the shores and the waves can double. The wave speed in open waters is 600-1000 km / h, but the speed drops sharply when the wave approaches the coast of 50-100 km / h.
During a tsunami, water recedes from the coast. The further the sea receded, the higher the waves. And naturally there are more options for destruction.