In my opinion, in your own home, you can do what you want - if you want to pee in your own shower, go ahead. If you're staying at someone else's house, a hotel, or otherwise using a shower that other people who aren't necessarily OK with it use, don't - just pee before you get in the shower (why you wouldn't do that in your home as well, I don't know - it would probably only save a few seconds to do it in the shower instead. But like I said, it's your house, you can do it if you really want to). In your backyard, technically it's also your property, but I think if anyone else sees you doing it, you could get in trouble for "indecent exposure." I also don't know why anyone would do that when there's a perfectly good toilet indoors, which would probably only take a matter of seconds to get to. In the ocean is also fine - there's already a bunch of pee and stuff in there. Where do you think the marine animals pee? In a pool, though, no, that's gross. People swimming in the ocean know there will be some gross stuff in the water, but in a pool, other people don't want to swim in your pee even if the chlorine kills any germs.
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Yet in my new wildness and freedom I almost welcome the bitterness of alienage. For although nepenthe has calmed me, I know always that I am an outsider; a stranger in this century and among those who are still men.
-H. P. Lovecraft, "The Outsider"