I can not see any water organisms! What am I doing wrong?

Let me guess: Either you used tap water (and in this case I really hope that you did not find any water organisms) or you used clear pond water. Most water organisms accumulate on the ground or on the surface of water plants or rocks. Scratch off some of the stuff for observation. Alternatively agitate the ground of the pond water a little bit. For safety reasons, you did not observe waste water anyway. Sewage water contains many bacteria, which are difficult to see and which are not so interesting and also possibly hazardous.