Grog was rum with water added. The mixture, unlike pure rum, would spoil, preventing the crew from hoarding rum to get drunk on. The barrel behind it is the butt of water, which was dispensed with a wooden drinking scuttle. To this day, "scuttlebut" is Navy slang for talk and rumors, which in old days tended to originate when the crew gathered around the butt and passed the scuttle to drink.