Jumping into open water might not be as safe as you think it would be. First of all, you don't know what's in it. And more importantly, you don't want to catch some disease from the water itself. To make sure, all you have to do is know which insects to look for, according to researchers Pilar Carreras over on the Wired How-To-Wiki page.

Why would you want to do this? First of all, you might catch some kind of nasty illness, and most commonly it's diarrhea, swimmer's ear and skin irritations.
Stoneflies (Order Plecoptera), Mayflies (Order Ephemeroptera) and Caddisfly (Order Tricoptera) are as suitable swimming pals as dolphins. Trout fishermen have been using these species as an indicator of good fishing for ages. These insects need healthy water to survive, so just by finding some of them in your recreational site, chances are high that the water quality is good.
If these insects are hanging out at the water hole then chances are its safe to hop right in. Head over to Wired How-To-Wiki for more: Wired How-To Wiki