Water is literally life, particularly in a survival situation. With about 60% of the human body comprising fluids, a loss of a few percentages can result in extreme distress, sometimes even death. However, without knowledge about the purity of the water found in the wilderness, it is a good idea to treat all water as contaminated, however clean it might look, and however thirsty you might be. Boiling is the one way to ensure that the water becomes good to drink. But there are a lot of ways by which to purify water and make it good to drink. And there are other ways with which to make the water drinkable, even though it might not be 100% clean.
When you consume contaminated water, waterborne diseases are transmitted into the body resulting in a host of illnesses, many of them that can really run you down making you weak and unable to function, sometimes for weeks and months. And not just through drinking, you wash your hands or wash your food and bingo, the germs enter your system.
Some of the more common waterborne illness are amoebiasis, giardia, e coli, dysentery, typhoid, gastroenteritis, SARS, hepatitis A, polyomavirus, and many more whose names are difficult to pronounce, but the illness can take you down for a long time.
A lot of people might go haring off looking for food, not realising that water has priority over food. Your body has enough food reserves to last nearly a month. But you will not survive without water for more than a couple of days. Loss of water in the body can lead to a host of serious health hazards. So you need to replenish your water resources first. You can never be too careful with water, particularly with habitation penetrating even remote areas. The rule is to treat all water as contaminated. Even one drop of contaminated water can make you sick for a very long time, often debilitating. There are bacteria, viruses, protozoa and intestinal parasites. You might end up swallowing a leech, which will then latch on to some part of your body and suck your blood ... from the inside. All extremely harmful conditions for the human body. So, purify the water you are going to be drinking as much as possible.
In the olden days one could drink straight from a mountain spring or waterfall. Unfortunately, these days habitation is reaching remote areas and there are villages or shepherds in far flung areas, often up in the mountains from where the water is coming from. They use the water for cleaning dishes, washing clothes, bathing their animals. And there is a lot of human and animal waste in the water, the source of many diseases. Faeces, human and animal, is a major carrier of disease. As far as you can, avoid drinking water straight from the source, even if it looks clean, refreshing and inviting.
If for some reason there is absolutely no way you can purify and clean the water for drinking, follow the old principle accompanied with a fervent prayer ... grit your teeth and strain out the big stuff. Not advisable.
In an OTA Survival School Course, you will learn of many different ways by which to filter and purify water, including some portable equipment that can be carried to purify water.