Gee george, that a hard one, how would you tell her.
Don't tell her , drug her ^__^
Relationships are about communication but that is a tough one. How about using the internet to open the subject by showing her a watersports site that you found "by accident." And there are many of them so this has become fairly widespread.
How I accomplished your quest..... Well she was really drunk so that made it easy. She also had to pee really bad so she didn't care where it went as long as she could ease up on all the squeezing it was taking to hold in. We were outside in my back yard long after dark and she was sitting on my lap kind of and she was so drunk I doubt she could of found her way to a toilet without my help so after one of her threats of peeing right there any second I said "fine already, let it gush," and within about 2 seconds she was peeing all over the two of us.
or just tell her