Water temperatures on a warm day
When the temperature decreases (A) closer to the bottom, the sonar beam is deflected down as indicated by the illustration. The school of fish (B) you thought you would see will not be visible at all.
Water temperatures on a cold day
When the temperature decreases (A) closer to the surface, the sonar beam is deflected up as indicated by the illustration. The school of fish (B) you thought you would see will not be visible at all. Instead you will probably see a ring of surface echoes.