Why do ships in the open ocean not notice passage of a tsunami wave?

Seismic waves moving through the ocean do not form tsunami's until they approach land. At that point the forward motion of the waves pile up on the land and form a series of forward flows that do not have an opportunity to recede due to the following waves. As a result, the waves keep coming on top of the previous waves and come farther inland. Ships at sea do not notice the tsunami until they are over the continental shelf.