Just ran across this in a wiki chapter on expressions peculiar to Mexican Spanish. Hehe.

Aguas!": Literally "Waters!" means "(Be) careful!" or "Watch out!"; it traces its origins back to the 1800 when toilet water was thrown through the window, people shouted ¡Aguas! so that other people in the street could be careful.

Sounds suspiciously like this "origin" for "loo" which is generally considered a folk etymology. Of course I could be completely wrong about that. It wouldn't be the first time.

I've heard aguas meaning 'watch out; be careful' in Mexico, and I've also heard the etymology before. It seems to be universally known. The Diccionario de la Lengua Española from the Real Academia Española lists the meaning (link). The etymology that one hears for loo which is slightly more believable is from French lieu 'place' which sounds like a kind of euphemism.

I wasn't doubting the meaning for a moment. I was just contemplating the etymology, especially in light of the supposed "loo" etymology which I don't believe for a moment.