The association of rum with the Royal Navy began in 1655 when the British fleet captured the island of Jamaica. With the availability of domestically produced rum, the British changed the daily ration of liquor given to seamen from French brandy to rum. While the ration was originally given neat, or mixed with lime juice, the practice of watering down the rum began around 1740.
To help minimize the effect of the alcohol on his sailors, Admiral Edward Vernon directed that the rum ration be watered down before being issued, a mixture which became known as grog. While it is widely believed that the term grog was coined at this time in honour of the grogram cloak Admiral Vernon wore in rough weather,the term has been demonstrated to predate his famous orders, with probable origins in the West Indies, perhaps of African etymology.
The Royal Navy continued to give its sailors a daily rum ration, known as a "tot," until the practice was abolished after July 31, 1970.
Today the rum ration (tot) is still issued on special occasions by H.M. Queen Elizabeth II order "Splice the mainbrace"! Such recent occasions have been Royal marriages/Birthdays, special anniversaries. Splice the main brace in the days of the daily ration meant double rations that day.