I'm a bit of a fanatic when it comes to reading about the First World War and lately, I've been extremely interested in the war at sea.
In Robert K. Massie's book Castles of Steel, he mentions, I think it was Jellicoe, drinking a thick cup of cocoa during the night action phase of the Battle of Jutland.
I'm currently reading Baltic Assignment - British Submarines in Russia 1914-1919 by Michael Wilson. In one passage he is describing the passage of E18 through the Sound into the Baltic Sea. E18 at one point has to submerge quickly, in very shallow waters (depth gauge showed 10ft). This passage describes what Lieutenant-Commander R. C. Halahan did next.
As the looks of dismay and alarm spread over the faces of the control room crew, he turned to one of the sailors nearby and said in a voice utterly confident and calm, "Get me a cup of cocoa, will you, Welsh."
The use of cocoa in navies does not seem to have been limited to the British, as I found in a mid-1918 doctrine memo by Lieutenant J. C. Van de Carr of the United States Navy a passage about food and provisions onboard U.S. Submarines. J. C. Van de Carr states that Cocoa should be served instead of coffee on night watches. I have a feeling this practice was adopted from the Royal Navy, as many other practices he lists he admits had been based on information given by the British in their experience.
So I guess my question is why cocoa was the drink of choice, for at least night-watches, onboard ships (specifically in the Royal Navy)? When did the practice start?
Cocoa, hot chocolate, or Kye, as the sailors called it, formed, along with tea and rum, a holy trinity of drinks that kept the Royal Navy running in the 19th and 20th Centuries. Cocoa was first issued to the RN in 1824, as a way of compensating for a reduction in the rum ration. It was issued, throughout the period, as a thick slab of chocolate. This was shaved or grated into a kettle, where it was boiled with water and condensed or evaporated milk. This produced a thick, sugary, warming drink. As you note, it was much praised as a drink for watchkeepers and to calm nerves in difficult situations. This seems to have been a result of its sugar content, and its thick nature, which ensured that it had to be savoured over time, rather than being drunk swiftly, as tea could be.