They're on one of the safest place you can be during a tsunami. Out to sea and on a ship. It's basically a small wave in the ocean but when all of that displaced water hits the shallows is when it becomes dangerous.
I am far more worried about the people in Japan, Hawaii and the west coast than the folks on Sapphire. They'll have a nice night's sleep and they're biggest concern is breakfast in the Horizon Court or the dining room.