This is a really interesting and informative thread for me.

In my own story, I've been rather sloppy so far in my use of different words for urine so far, but now that I think about it, my female characters usually say 'pee' and my male characters say 'piss' (I use a variety of words for both the substance and the activity in the non-dialogue parts). I should probably be a bit more rigorous about that in the future installments.

The collection of slang for urinating will also be helpful (though not as much, since that list is pretty male-oriented ).