While not as likely to happen to you on solid ground as, say, being hit by a meteorite – falling off of a cruise ship has happened more than once.  First to Larry Miller:

after he fell off the cruise ship Carnival Inspiration, he had been hit by a passing boat, then managed to climb onto a marker buoy – TBO.com

But it’s happened again!  Less than a day after Larry Miller was rescued, another person has gone overboard, this time in the Gulf of Mexico!