Another kind of play, also known as “water sports”. Basically, some people enjoy urinating on their lovers, or having their lovers piss on them. Pissing is really a very intimate thing; your urine is a part of you, it’s warm and wet, it feels good to let it out, it comes from your genitals. Some get a thrill of power from having someone bound beneath them who can do nothing but take it as the shower lets loose; others get off on being made to pee, to wet their pants, it’s naughty and they need to be punished for it.
Safety-wise, urine is essentially sterile; it’s not necessarily free of HIV, so it’s not safer sex to drink someone else’s urine. Also, urine contains mostly salts that your body is trying to eliminate, so drinking it again will strain your kidneys. If you’re drinking urine, make sure to drink lots of water as well.
Some people are into scat play, which is playing with shit. I don’t know any who are, but they’re out there. Scat is obviously even less safe than water sports; in particular, hepatitis and intestinal parasites can be spread by oral contact with even a tiny bit of feces. People who enjoy rimming (oral-anal contact) should be aware of this, and clean themselves VERY thoroughly at the least, although even thorough cleaning will not eliminate all risk. For more on this, see the next question.