A tsunami is an enormous amount of moving seawater that has been disrupted when an earthquake takes place in the ocean.  The movements of pieces of the earths crust, as they converge against each other, suddenly on the ocean floor, create  an enormous shaking earthquake effect  that displaces the seawater, because one piece of crust rises and the other one sinks.    It takes place in seconds but with the force of an explosion.  The water has to go somewhere.  Just like throwing a pebble into a stream, the ripples go out in all directions at once.  All the energy from this collision is transferred to the water, and huge waves rise up and carry that water outward as far as it can freely flown until the energy is spent.  Anything that gets in the path of the walls of water is swept away.