There is also the problem of rogue waves. Rogue waves can even occur in so called calm seas, although even the Gulf of Mexico can sometimes have some choppy seas..

http://en.wikipedia.org/wiki/RMS_Queen_Mary

Quote:
In December 1942, Queen Mary was carrying 16,082 American troops from New York to Great Britain, a standing record for the most passengers ever transported on one vessel.[20] While 700 miles (1,100 km) from Scotland during a gale, she was suddenly hit broadside by a rogue wave that may have reached a height of 28 metres (92 ft). An account of this crossing can be found in Walter Ford Carter's book, No Greater Sacrifice, No Greater Love. Carter's father, Dr. Norval Carter, part of the 110th Station Hospital on board at the time, wrote that at one point Queen Mary "damned near capsized... One moment the top deck was at its usual height and then, swoom! Down, over, and forward she would pitch." It was calculated later that the ship tilted 52 degrees, and would have capsized had she rolled another 3 degrees.[21] The incident inspired Paul Gallico to write his story, The Poseidon Adventure, which was later made into a film by the same name, in which Queen Mary depicted SS Poseidon.