The idea of 'tapping the Admiral' is, in my opinion, just pure urban (or naval) myth. Spirits were always guarded by Royal Marines, and drunkenness was punished severely. The alcohol provided by official issue was enough to ensure that any substantial extra (brandy was stronger in those days and spirits of wine almost pure alcohol - equivalent to IMS (Industrial Methylated Spirit) would leave the recipient pretty well comatose.

Unfortunately, many of these old rumours are being perpetuated by the Royal Navy, who as a body are pretty poor historians.