“When I was a young subaltern in the South African War, the water was not fit to drink. To make it palatable we had to put a bit of whiskey in it. By diligent effort I learned to like it.”

It seems likely that the attribution to Twain is apocryphal. It is not listed as authentic on Twainquotes http://twainquotes.com/, and is not listed at all in either R. Ken Ramussen's The Quotable Mark Twain (1998) or David W. Barber's Quotable Twain (2002)
Misattributed

Reflections on his earlier life, written when he was 27 (December 1862), published in Letters and Journal of W. Stanley Jevons (1886), edited by Harriet A. Jevons, his wife, p. 11.
Context: When quite young I can remember I had no thought or wish of surpassing others. I was rather taken with a liking of little arts and bits of learning. My mother carefully fostered a liking for botany, giving me a small microscope and many books, which I yet have. Strange as it may seem, I now believe that botany and the natural system, by exercising discrimination of kinds, is the best of logical exercises. What I may do in logic is perhaps derived from that early attention to botany.

Source: Mercury's War

What We Talk About When We Talk About Love (1981)

World of Colm Tóibín, writer http://www.telegraph.co.uk/culture/books/authorinterviews/9108553/World-of-Colm-Toibin-writer.html, The Daily Telegraph (27 February 2012)

The Diary of a Siege, 1900, H.W. Nevinson.
In conversation with Henry Nevinson, October 2, 1899, at the outbreak of war. According to Nevinson, Joubert spoke English with a "piquant lack of grammar and misuse of words".

Source: Hard-Boiled Wonderland and the End of the World

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