Urine is not poisonous, and is well diluted in a pool. You can swim without any worries. After showering you might like to disinfect the bottoms of your feet, as there you can pick up skin infections.
As for the lakes, wild birds spread most disease epidemics to Europe, and there I would be careful not to swim in stagnant lake water.
http://www.youtube.com/watch?v=syiASljau5w