One of the ways that we cooled off was swimming in the canal or ditch (whatever you want to call it). There was a canal that ran just behind our street, the Phyllis Canal. Our dad bought life vests for us and we all used to tube down the canal along about a quarter mile stretch. Then when we got older and were good swimmers we would go there on our own. It was a great way to keep cool. I know that canals are dangerous and I do not take my kids floating, like my dad did. But, when I was growing up- it was fun- we were safe- and we were smart.
There was also a place called "the ropes" a mile or so from our house where a few irrigation ditches converged. The trees had grown over and shaded it perfectly. It was only four feet deep or so and all sand on the bottom. It made the perfect place to swim and swing from the ropes that people had hung there. It was just the way you imagine it. Cool water- a shaded area - great summer fun.
Enjoy the hot days!
FYI- for disclosure sake- here is a PSA about canal safety