The Yale Book of Quotations cites as its source for this quote Graffiti: Two Thousand Years of Wall Writing, by Robert Reisner, 1971. But W.C. Fields’ granddaughter says he didn’t exactly say the last part of the quote: what he said was, “I don’t drink water. You know what fish do in it.” See her comment below.
Here’s another wonderful Fields quote (although don’t think in terms of present-day Afghanistan): “During one of our trips through Afghanistan, we lost our corkscrew. We had to live on food and water for several days.” (from the 1940 film, My Little Chickadee.)
See http://wcfields.com/ for a biography, filmography, and more about W.C. Fields.