MORTIMER HOWELL COBB, 81, who survived the sinking of the Titanic and later made the seas his livelihood, died Monday after a bout with cancer. Cobb was 7 years old and traveling with his governess when the British steamer Titanic sank after hitting an iceberg in the North Atlantic on its maiden voyage in 1912. Of 1,500 passengers, about 700 survived. Cobb later studied at Phillips Exeter Academy and Williams College and became a captain in the U.