This morning, I davened in a shul that I don't normally daven in, and I noticed that they had some siddurim that I don't usually see around. So I decided to do some research for your question, and here's what I found:
Siddur Tehillat Hashem (fourth edition, April 2004) translates מי רגלים as "water of Raglayim," and cites the gemara Kerisus 6a-b (both sources already cited by Menachem), as well as Yerushalmi Yoma 4:5. (page 243, footnote 1)
The Metsudah Shabbos/Yom Tov Siddur (fourth edition, 1984) also translates מי רגלים as "water of Raglayim," and cites Kol Bo as explaining that there was a well that was called "Raglayim," whose water was good for this purpose, but couldn't be brought into the Temple, because מי רגלים is also a euphemism for urine. They also say (also in the name of the Kol Bo) that מי רגלים might actually mean urine. (page 173, footnotes 10-11)
Artscroll, as mentioned in comments, does actually translate מי רגלים as urine, as does the Koren Mesorat HaRav Siddur.