A U-Boot is a beer cocktail (popular in Germany, Poland, and Flanders) that is made with a glass of beer and a shot glass of vodka. In Germany, the liquor korn is sometimes used instead.
It is called a U-Boot (German abbreviation of Unterseeboot, "submarine") because the shot glass of vodka sinks to the bottom of the glass of beer. The shot glass then "surfaces" when the cocktail is drunk.
In Flanders, it is called a "Duikboot", literally translated "Submarine."