Josephus Daniels (18 May 1862-15 January 1948) was appointed Secretary of the Navy by President Woodrow Wilson in 1913. Among his reforms of the Navy were inaugurating the practice of making 100 Sailors from the Fleet eligible for entrance into the Naval Academy, the introduction of women into the service, and the abolishment of the officers’ wine mess. From that time on, the strongest drink aboard Navy ships could only be coffee and over the years, a cup of coffee became known as “a cup of Joe.”
From the Fleet Forces website.
Ha, Phrase Finder had cuppa joe in the discussion boards:
“It may be a variant of “java,” also a popular term for coffee since the 19th century, when the island of Java in Indonesia was a major source of the world’s coffee.
There is/was a company here in NY named Martinson’s Coffee which was owned by a man named Joe Martinson. The whole neighborhood would have the fantastic aroma of coffee roasting and there came the name a cup of joe.”