The Lusitania
During the years of WW1 many great ocean liners were sunk by German U-boats, whether they were troop carriers, hospital ships, or passenger liners, it didn't matter, if they were flying a flag which belonged to Britan, they were considered the enemy and sunk.
Perhaps the greatest loss of a passenger liner which was still a passenger liner during the war years was the Lusitania. She was owned by the Cunard line, and was in service for the fabled South Hampton to New York route.
1st of May 1915 she left New York heading for South Hampton carrying 1,965 passengers and crew ( the majority being American citizens who's country was not yet fighting in the war ). 7th of May she had been making good time on her run, and while off the coast of Ireland a German U-boat spotted her, lined her up in their sights and fired a torpedo. The torpedo wasn't spooted til it was almost upon the ship.
The topedo hit the Lusitania putting a large whole in her starboard side which water began rushing into the 1st and 2nd boiler rooms. The ship was then rocketed by a 2nd explotion, this time coming from the eternal area's of the ship, ( some believed the Lusiatania to be carrying explosive materials, but recent studies have shown that this second explotion may have been caused by the extensive gases which had built up in the ship, from the coal ).
The Lusitania took a mere 20 minutes to sink taking 1,201 people with her -- only 764 survived.
