In less extreme scenarios (i.e 99.9999% of the time), water is a non-compressible substance that, at 8.34lbs-per-gallon, is heavy and you'll need to be drinking at least a gallon of it a day when you're outside. You can't physically carry enough water to see you through a multi-day trip, so you'll need to find it along the way.
Water Bottle: A plain, unpainted, uncoated, unlined aluminum or steel Klean Kanteen-style water bottle works best. Not only do they allow you to carry water, but you can easily boil water in them too. Carry a large enough container for your needs or multiple containers. I generally carry two 27oz bottles or a three-liter hydration pack.
Rubber Hose: If you'll be travelling through places where water can be hard to find, a few feet of ¼ inch plastic or rubber hose can help you draw it from difficult, otherwise unreachable sources.
Hand Sanitizer: Prevention is as effective as a cure. The most common water-born pathogens are bacteria that live in poop. You need to worry about contamination from yourself as much as you do from other people and animals.
Plastic Trowel/Shovel: Go to the bathroom at least 200 feet from any water source and bury your feces at least a foot deep. Doing so will keep your germs out of the water. A little shovel can also help you dig for water.
Fire Starter: Carry a non-mechanical, non fuel-dependent fire starter such as a ferro rod. Those will never brake, work regardless of weather conditions and will never run out of fuel.
Clear flowing water coming from somewhere without people, manmade things or obvious signs of pollution is best. If you come across a spring or stream while outdoors, take advantage of it and top off your water bottles.
Lakes, ponds and rivers are less ideal. The first two are stagnant, which may mean increased levels of bacteria and other gross things, while large rivers are typically full of pollution. Be especially wary after any flooding or if the river flows from or through a population center, under a road or around any construction, chemical plants or similar on its way to you.
Snow and ice (so long as it's not sea ice!) can provide a good, readily available source of clean water in the winter. Never eat snow or ice though, doing so will lower your body temperature while not doing as much for hydration as melted water will. You're still supposed to purify snow after you've melted it, but so long as it's not black, yellow or brown, I've never bothered. To melt snow, put it in a container with a little water in the bottom and bring that up to temperature, adding more as the snow melts. Just throwing snow in a hot pot will make it taste awful.
You can also filter water from mud or dig for it in dry river beds or other low lying areas. One thing I haven't tried is creating a "beach well" or "swamp well" by digging a hole and shoring it up a ways back from the shoreline. Basically, there tend to be acceptably clean water underground around bodies of water or where they sometimes are.
Never drink sea water or urine, but if they're all you've got, you can boil them and collect the steam with a plastic sheet or bag. A solar still works similarly, but much, much, much slower. You can make one of those again from a plastic sheet or even two water bottles.
You can also ask friendly foliage to lend you a hand. Wrap branches in plastic and, over time, condensation will form and small amounts of water will collect at low points in the plastic, which you can define with small rocks. Never do this with a poisonous plant.
If you can't find a water source, start walking downhill (also a good way to get un-lost) and look for dark patches in the landscape (especially on rocky hills or faces) and any group of vegetation that stands out in a low area. Really though, don't put yourself in the kind of situation where you need to find water. Plan trips in areas where it's available or, if you're traveling through the desert on a dirt bike or something, map out where it's available ahead of time. A little bit of planning and you'll never find yourself hosting the kind of reality show where you have to drink you own piss.
Boiling: The most effective way to remove both viruses and bacteria from water is simply to boil it. Bring it to a roiling boil and keep it there for 60 seconds or so and you've got safe drinking water.
A metal canteen, pot or cup is the easiest way to achieve this, but in a pinch you can boil water in plastic, bark or even paper so long as you're VERY CAREFUL and make sure the container stays completely full.
Filtering: There's a million filters on the market, but they all fall into two categories — one's equipped with carbon or ceramic filters that remove gross stuff and bacteria or ones that also treat the water with iodine or another chemical to kill bacteria. Filters are complicated, heavy expensive and need to be replaced often. For that reason, I've never bothered with them.
It can be a good idea though to filter water before you treat it by boiling or other methods. Just to remove the obvious gunk if you're drawing from a gross source. A T-Shirt or sock or similar will work in a pinch, but a paper coffee filter works much better and stuffing a couple into the bottom of your pack, just in case, adds no weight.
Chemicals: The most effective and affordable way to purify water is simply to add a couple drops of Tincture of Iodine 2% to your water bottle. Make sure you're buying "Tincture of Iodine 2%" not some other substance claiming to be iodine. Iodine's also the active ingredient inthose little purification pills you add to your water bottle. Those are great too, but cost more and the same size little bottle won't last as long as just the tincture. This kills viruses and bacteria and anything else that may be in there. Bleach also works, again just add two drops or so to a water bottle and shake it up.
UV Lights: Steripens and other battery-powered devices exist that purify water by treating it with UV light. They kill both viruses and bacteria, but work best if you filter the water you're treating first.
Solar Power: If you're really stuck out there and have no other means, leaving a clear water bottle in the sun for a full day (if sunny) or two days (if cloudy) will kill bugs in the water via UV radiation. This works well in very large quantities, such as cleaning water for a family or community after a disaster. Just fill up dozens or hundreds of clear water bottles with filtered water, make sure you remove their labels and leave them out in the sun.
You read about solar stills in survival manuals, but you should really hope you never need to rely on one. They take forever to create even the smallest amount of drinkable water. Basic precautions and preparations should mean you'll never need to make one.
After purifying water in any of the above ways, you'll need to get any remaining gross water off your water container. Turn it upside down, then slowly and carefully open the top a little bit until water "bleeds" out, cleaning the spout and surfaces you'll touch with your mouth when you drink.
Just Drink It
If I'm way out in the wilderness somewhere and can say with reasonable certainty that other people haven't been around to poop near a water source, a lot of the time I'll just drink the water without filtering or treating it. I've only ever caught Giardia once and that was at a place where I definitely should have been cleaning my water, but got lazy. It manifested itself with a minor, three-day-long case of the runs a week after I got back from the trip. I'd imagine that I've developed something of a resistance to waterborne-illnesses though, I've been drinking it untreated all 33 years that I've been on this planet.
Pack the materials to make your water safe. Tincture of Iodine 2% is so cheap and easy to use that there's just no excuse not to. Boiling takes a little while longer, but is even more effective. But, if you're in the outdoors and you have to chose between drinking potentially dirty water and going thirsty, drink the water. Dehydration will kill you much, much faster than a few bacteria ever will.
Source: gizmodo.com by Wes Siler