WHEN YOU SMELL SOMEBODY BEHIND YOU, AND THERE'S NOBODY THERE. According to Admiral Gallery, that's when it's time for a submariner to hit the showers. Modern submarines might have nuclear power plants, with diesels used only for auxiliary power, but the paucity of showers and the depletion of fresh foods are still realities in the submarine service. The nuclear power plant means "on patrol" equates to "submerged," thus no more opportunities to do a bit of fishing while the diesels are recharging the batteries.
An aside: much of The Hunt for Red October appears to draw on Admiral Gallery's memoir.