Blow up subs, but don't sub the subbed subs for subs.*
*Feel free to replace every instance of "sub" with "submarine sandwich" or whatever your region names them.
The looks of Gremlin's Depthcharge release and Taito's Sub Hunter are slightly different, the palette for the High Score text and the submarines is the same while it was reversed for the HUD and the player ship. Also, the large black-colored layer that represents water in Depthcharge is not in Sub Hunter, probably done so that the artwork on the cabinet's screen would do this job instead.