When a great earthquake ruptures, the faulting can cause vertical slip that is large enough to disturb the overlying ocean, thus generating a tsunami that will travel outwards in all directions.

If you are at sea during an earthquake, your experience depends on the depth of the water and your proximity to the epicenter. In the deep, open ocean, you may not feel the earthquake at all, and a resulting tsunami would only be a small, unnoticeable ripple beneath the hull. However, in shallow water or near the coast, the energy can manifest as a "seaquake," which feels like the ship has struck a solid object or is vibrating violently. For scuba divers, an earthquake can cause sudden changes in water pressure, loud acoustic "booms," and intense "silt-outs" as the tremors disturb the seabed. The primary danger for anyone at sea is the tsunami that may follow; ships are safest in water deeper than 100 meters, as the waves only "pile up" and become destructive as they reach the shore. If you are on a boat near the coast when an earthquake strikes, the standard procedure is to head immediately to deep water and stay away from harbors or narrow inlets until the tsunami warning has been officially cleared.