One method I like using in the field to help minimize the larger debris and wrigglers found in foul water is called the Indian Well. This is a method still in use by the Bushmen in Africa. Simply dig a one foot deep hole about one foot away from the water source (riverbank or lakeshore). Allow the water to seep through the walls of the hole and then mop it up with a bandanna. This is not "purified" water but the sand/soil between the watersource and the hole will act as a sieve to remove the larger critters and it sure beats sucking down pollywogs.
The cowboys out my way in Arizona have a good method for drinking from backcountry water sources. They say, "Just grit your teeth to strain out the chunks."
I should preface this next statement by saying it occurred when I was younger. For nearly twelve years, I never purified water. I drank straight from streams, lakes, and cowholes while on long survival treks. I never had any problems. Speaking about this with my doctor once, he said that I probably had "chronic giardia." He used to have it while working in the Peace Corps years ago.
Now I carry a small bottle of Potable Aqua Iodine or boil water for one minute. If you have the means of treating backcountry water, then do so. Using a good water filter, iodine tablets, or boiling is always recommended, but remember that there is a cure for giardia and waterborne illnesses. There is no cure for death from dehydration!
If you can't properly purify the water then drink up and go see your doctor after your rescue. Stay hydrated (which means peeing clear fluid) and stay cool by soaking your clothes.