We had this happen when we were on a "cruise" -- well, pimped ferry ride really, typical for the Baltic sea -- but luckily it was aborted much sooner than 50 hours. Three or four ferries were searching an expanding area near a point where somebody had reported seeing somebody fall overboard from our ship. After six hours they had accounted for every passenger and every crew member, and we continued to Riga. We had to optimize our visit pretty hard to fit our 8-hour sightseeing into 2 hours to catch the return ferry. :-)