Where do you think all the raw sewage through out the world ends up? In our oceans.
Peeing in still water, like a pond or lake would be bad because their is no fresh water moving in and out of it. Plus, ponds and lakes are fresh water (without salt). Salt has a way of cleaning impurities (to a certain extent), like in oceans.
Lakes and ponds do not have fresh water inlets and outlets, they relai on the flora of the soil and rock beneath and around it to keep it clean enough for animals, however, it is easily spoiled by bacteria and parasites, which would cause it to be stagnant making it stink and filty, for humans and animals alike, so peeing in these small bodies of water would not be good.