Paul, they are mentioned in The SAS Survival Handbook. The author (Wiseman (?)) said he was never able to verify their existence or abilities, but the way to piss was to interrupt the stream every few seconds. That can be done with no discomfort by changing one's aim and spraying at a mate or a tree, etc. They are purported to be in parts of South America.

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I went googling for the creatures name and lo, there IS such a fish. The video talks about it.

http://animal.discovery.com/videos/river-monsters-fish-swims-up-urine-stream.html