I really hope you read The Gear – Water post.
But just in the event that you were stupid enough, sorry but have to categorize it that way, to forget clean water, here are a few tips. Before leaving for a deployment, read on how to find water in different places in the world, specially if you know where you are going.
In places where there is forestry you will find “vines” and they are full of clean water. Grab the vine as high as you can and cut it keeping the bottom part in your hand. Then cut further below, which is where likely you will drink it from unless you are putting it in a bottle. Fortunately where there is one vine, there are many. Look for YouTube videos on this. The top part will also start dripping so catch some of that too.
In winter places hopefully you can find ice or snow, don’t drink the yellow one, or for that matter anything that is not white. If possible please pass it through a filtration straw or similar.
In the tropics or if near a beach, look for bamboo and coconut trees. Bamboo being the easiest to find in many places has tons of water inside. Bamboo has rings which actually divide it into segments. If possible cut a long piece or whole bamboo tree down, then next to any ring cut a notch in the bamboo, when you see it hollow, turn it and pour it into a container or your mouth. You do not need to filter this, the bamboo was your filter. You can keep cutting each segment as you need more water. Also look in YouTube for other uses of bamboo in survival situations.