The Key (1958)

"He had the life expectancy of a match."

In wartime England, circa 1941, poorly-armed tugs are sent into "U-Boat Alley" to rescue damaged Allied ships. An American named David Ross arrives to captain one of these tugs. He's given a key by a fellow tugboat-man -- a key to an apartment and its pretty female resident. Should something happen to the friend, Ross can use the key.