Waves, waves, waves!!!!

An ocean earthquake can generate a series of powerful water waves called tsunamis (tsu - harbor, nami - wave).  At sea, these waves may not even be felt by fishermen and sailors.  By the time a tsunami reaches land, however, it may have a height anywhere from 30 meters to 50 meters.  Sometimes they can become  mega-waves, reaching between 100 to 300 meters.  Why is it that ships out in the high seas don't feel the impact of tsunami waves, while coastal communities are destroyed by them???????