You're not going to want to carry all of your drinking water in to the back country with you. A friend of mine does this because he is a nut, and because he only comes on short trips. There's more than one way to get clean water from fresh water lakes, and drinking it directly shouldn't be the one you chose!
The main concern is the Giarda parasite which is transmitted from animals to you by their poop getting in the water. If you get infected with Giarda you can get any of these fantastic symptoms:
- stomach cramps
- unusual gassiness
Act today and we'll throw in weight loss for free!
You aren't going to want any of that when you are a 20 km paddle from the nearest road, not counting the portages. It's not brief either - it lasts from 2 weeks to 4, roughly.
The CDC has much more detailed information on water born pathogens.
Boiling won't clean your water of dirt, squirrel farts, or the oil slicks left by those inefficient two-stroke motor boat engines. It will at least make the living things in the water dead things, which is good if you want to avoid the Giarda parasite. Let your water boil for a good minute to make sure everything is good and dead. A buddy of mine likes to use his shirt as a filter to pour the cooled, boiled water into his drinking bottle, which is better than no filter but not that great.
You can cool your boiled water faster by storing it in bottles (that are okay with 100 C water) and leaving them in a nearby lake. Lakes tend to get cold fast as you get deeper, and you can play with how much water you fill them with to change their buoyancy.
This method is easy, but it is slow and uses fuel. If you boiled dirty water it's still dirty. The water you get is hot, which is not great for drinking water. As a plus, it kills everything. Viruses, bacteria, parasites, it kills them all.
This is the method that I use. Make sure you chose a 1 micron filter, as the junk you're trying to filter out is very small.
I use a Katadyn Vario filter to remove nasties from my drinking water. If I'm canoe camping, I take a container that fits a good amount of water (mine can store 15 L) and paddle to the middle of the closest lake. I fill it up directly with the filter, take in the scenery, and I'm done. Cold, fresh, drinkable water. You can screw it to the top of most water bottles and the filter is replaceable. I find I have to replace the filter about once a year. It gets noticeably darker and water gets much harder to pump when it's dirty.
When using a filter, be sure you are pumping the cleanest water you can find, hence my going to the middle of a lake. This is much harder when you are camping on land without a canoe. Near the land the water will have all kinds of bits of stuff in it that will clog up your filter quickly. I've had my filter clog up in one trip after a friend used it in the boggiest, grossest water they could find.
After about 4 years or so I find the pumping system becomes less reliable and I frequently have to take it apart, clean it thoroughly, and re-grease the o-rings. It's a pretty simple mechanism, and I haven't quite figured out what part isn't doing its job right any more. Four years isn't too bad, though.
The future is here! You can now purify your water with light invisible to your own eyes. However, that's a big minus for some people. You can't really tell that the thing is working except by trusting the progress indicator on the thing. With a filtration pump, you feel the resistance of your filter and can watch it get dirty over time. The SteriPen is the best known model. I haven't used it, but I do trust that it works on a scientific level. I don't really like the idea of relying on batteries or access to a USB port for recharging, though.
Chlorine and Iodine
No. Giardia and other baddies don't care. Don't use it.