A few years ago, we were on a cruise when the cruise director commented that the dancers on stage were sometimes challenged by rough sea conditions. "When they leap up into the air," he said, "they never know if the ship will be in the same place when they come down."

The bad news is that the theatre is usually located forward, the part of the ship that has the most motion in rough water. The good news is that cruise ships are really big, and have stabilizer fins. This means that they are relatively stable in heavy seas.
Howard