There are two things to keep in mind when thinking about water. What to keep it in, and how to get it. Now sure, you can and will pack water with you, but if you run out, you are going to have to get more and that may require some way to treat it for consumption. Not to mention, water is heavy at about 8 pounds per gallon.
First let’s look at containers. Canteens are cool, but I prefer Nalgene Bottles. They are virtually indestructible and they do not retain tastes or smells. Recently, my brother made a nice stiff rum and coke in a Nalgene bottle (not full, of course, that could have gotten ugly!), and when he was done with it, I cleaned it out and drank water from it. I could not smell or taste the rum or coke at all. It was pretty impressive, so I highly recommend them. Plus, they have measurements on the side so they are good for measuring out liquids. They can also withstand freezing and boiling temperatures. That does NOT mean you can boil water in them, it just means you can pour boiling water into them if you so desire. (Well, you could boil water in them, but you better know what you are doing and do not set the bottle directly over the fire!) You can really get hooked on Nalgene bottles with all the things you can get for them and use them for. You can even get a coffee press for a Nalgene bottle!
Another thing to consider is if your survival bag is specifically designed for a water bladder. If it is, I say get one and put it in it! Water is the most important commodity you are going to need so the more storage the better. You aren’t going to want to carry around a 5 gallon bucket of water, of course, but a 100 oz water bladder for your bug out bag that is designed for it is a no-brainer. What I mean is, the bag is designed to allow you to situate the weight of the water as efficiently as possible, so it is probably your best means of water conveyance. I am personally looking at a Camelbak Water Bladder for my main bug out bag. I have a smaller knock-off hydration pack that I have used but I haven’t yet outfitted my bug out bag with one yet. Survival bags can be an on-going build if you really get into it! But from all the research I have done on them, the Camelbak water bladders seem to be the most durable and best option. Plus, they have a large opening for allowing you to clean it out easier.
Now we turn to water purification. This is extremely important if you have to obtain water on the go. That stream may look crystal clear, but there is most likely all kinds of bacteria and other nasties in it. One of the most common filters that I see going into bug out bags is the Katadyn Hiker PRO. The output is about a quart a minute and is good for about 200 gallons depending on the water you are filtering. There are also attachments for Nalgene bottles so that makes this real easy to use and fill up your water bottle. This has been one of the more popular ways of filtering your water. It doesn’t hurt to use the water purification tablets like Potable Aqua or something similar, but even if you use the extra tablets to remove the iodine taste, it still kinda tastes bad. But in the short term, they do work.
To kill the bacteria and protozoa that gets through a filter, you might want to look at a SteriPEN Adventurer. Previously, I would recommend a Miox Purifier but the new SteriPen Adventurer seems to be beating out the Miox in how fast it works and its ease of use. You can get one with rechargeable batteries and a solar charging case, but I would just pack extra batteries instead of spending all that money on a charging case. I haven’t used one of these yet, but I am leaning towards getting one for my Bug Out Bag. You will, however, have to filter out any large particulates in the water, so you will probably want a cloth or coffee filter for that. In a pinch, a bandana is pretty useful.