Seal of the United States Coast Guard.

576px-Seal of the United States Coast Guard Auxiliary

Seal of the United States Coast Guard Auxiliary.

The Coast Guard and Coast Guard Auxiliary were branches of the United States armed forces that were active in 1955 that performed rescues at sea. The Auxiliary was a support organization of civilian volunteers who were 17 years of age or older.

As Marty came downstairs from Lorraine's bedroom, her mother Stella asked how long he would be in port, and that she guessed he was a sailor because he wore a life preserver. Since she was the third person who had never seen a burnt orange down vest from 1985 (the first two being Lou Carruthers and Skinhead), Marty acknowledged her guess, saying "Uh, Coast Guard".

A short time later, when Marty met Emmett Brown for the first time, Doc tried to read his thoughts using the brain-wave analyzer he had just invented. After two unsuccessful attempts, Doc noticed Marty's vest, but thought he looked too young to be in the United States Coast Guard or its volunteer auxiliary, so made a wild guess that Marty was collecting donations for a Coast Guard Youth Auxiliary — which was either an amazing coincidence, or due to an invention that actually worked.