Many ancient civilizations have myths that include floods, this is attributed to some temporary phenomenon that occurred in some villages. The story of Noah and The Ark from the bible has a flood included but this just one of the many stories that include them. The Nile River, for example, flooded constantly the valley leaving a rich fertile layer in which Egyptians planted their crops. Egyptians believed that Sobek the crocodile God was responsible for this floods that gave them rich, fertile soil.
Another idea of why water is always being mentioned in myths is that all life came from primordial waters. One thing is for sure in myths that if water can bring life it can also bring death. Watch this interesting video and learn more about ancient myths in Mesopotamian culture.