It's not a goof. It's just a variation on an old joke. On the submarines I was on it was a tradition to show up for watch wearing life jackets for the first time a newly qualified Diving Officer of the Watch would be standing that watch. Probably what inspired this movie as much as anything else is that for all the extremely hard work, personal sacrifice, and even danger submariners undertake they have a tremendous sense of humor which takes the form largely of every kind of prank imaginable. (If you read about pranks on soundstages, etc., in Hollywood it can give you a pretty good idea. You could swear some of those guys must have been ex-submariners, or, at least, if they ever became submariners, they'd fit right in.) This is one of the milder ones.