Factual error: The film was released in 1958 but depicts a 1943 World War II mission in the Pacific by a US submarine capatain played by Clark Gable and his first officer played by Burt Lancaster. In two separate scenes the crew listens to a "Tokyo Rose" broadcast on radio and the background music is "Kiss Me Once, Kiss Me Twice, It's Been a Long Long Time" by Sammy Cahn and Jule Styne. The song was not released until September 1945, one month AFTER the end of the war in the Pacific in August 1945. The song was famous for celebrating returning troops and therefore also makes no sense in the context of 1943.
Factual error: When in his office after being sunk, Clark Gable and Don Rickles are together. The phone is in the background on the corner table. The phone in the office is a 1950s model rotary phone with push buttons for several lines. This was not available in 1942-1943. There is a wall phone hanging on the window frame that is also a 50's model with a 50's handle.
Answer: Sailors are very superstitious, they considered it a bad omen. Number 7 is an area of the ocean where most ships and subs have gone down.