Scratch is a another name for the devil.
Rum went by many names in the Royal Navy including rumbustion, rumbullion, and Nelson’s blood.
Each sailor was rationed a gallon of beer a day. Grog was a diluted rum drink, often blended with citrus juice to keep the scurvy at bay. Sailors received two rations of grog a day, amounting to two gills (half a pint) total.
To take the King’s shilling, representative of being paid by the crown, was to agree to enter service in the Royal Navy, often under the duress of a press gang. There are 20 shillings to a pound and an able seaman was typically paid a shilling a day in the early 1800s.