Water filtration may be mostly unnecessary, but water purification (elimination of gut-crunching pathogens) is another story. If you're in a true survival situation, taking a chance on drinking water that could have dangerous pathogens is probably not worth the risk. Every water source is different and carries a different risk profile (how far upstream, where the water is sourced, etc).

You don't need to buy anything to purify water. Oddly this article did not discuss the simplest method of all - just boil it. A steel, non-insulated water bottle works fine.