Cruise ships are actually safer at sea during a tsunami than they are at a pier. In the deep ocean, a tsunami is often just a long, low wave that a large ship might barely notice passing underneath. The real danger is in shallow water near the coast, where the wave builds height and creates violent currents. If a tsunami warning is issued while a ship is docked, the captain's priority is to "sorties" (evacuate) to deep water immediately, sometimes even leaving passengers behind if they are still on excursions. Modern ships are equipped with sophisticated monitoring systems linked to the Pacific Tsunami Warning Center. If they are already at sea, they will simply reroute to deeper waters (typically deeper than 180 feet) and stay away from the coastline until local authorities give the all-clear for the harbor.