Ben,yes, the old jokes are always the best but maybe my joke fails because it is two nations separated by a common language and that "passing water" just doesn't have the same mirth making qualities in the U.S.
I asked one of my sons who went to university in America if the term "passing water" for urination would be understood there, he said "it certainly would dad, in a population of sixty million there are plenty of piss artists" http://dictionary.cambridge.org/dictionary/british/piss-artist
Be careful that you don't get any water used by Lisette Model. She once said, "Darling, if you think my prints are bad then you should see my negatives."