Throwaway VS Reusable: I actually have both. Because my reusable does not fit in the side pocket of my day pack but also doesn't carry enough to be a good backup, I use it for what I call overflow or excess water for longer journeys. While I've been out circumnavigating the globe, I've had some long trips over 20 hrs. In those cases, I need to carry more water. The reusable bottle I keep in my Rucksack is empty when I don't
need it. It doesn't take up much room and is extremely light when empty. It has been a real lifesaver on those long legs. Throwaway bottles; I buy, at the beginning of my trip, two heavy-duty, strong throwaway bottles of water. As I said earlier; a big one, say two quarts, and a small one of say 16 oz. I've been traveling now for 4 months and still have the same bottles I bought back in New Zealand. They have served me well. Like I said, be sure to buy ones that are made well and are nice and sturdy so they will last you the entire journey. In the event something does come up where you lose them or have to throw them away, then so be it; they can be easily replaced.
Water and Air Travel: You can carry your EMPTY bottles through security. If you've forgotten to do so, just empty them at security and keep them. Then fill them up once you pass security. If you have to go through secondary security and have already filled your bottles and they want to take them from you, be sure to tell them you want to keep the bottles. Then once you get to your gate they should have a place to refill them...again. This happened to me at the airport in Singapore. I had a secondary security check, forgot to ask for all my bottles back, and had an 11-hour flight in front of me. I only had one bottle (my reusable one) for the whole flight. I was flying a no-frills airline that did not provide free water. Needless to say, taboo or not, I dropped some cash for the water that day. It was that or die of thirst and I wasn't done adventuring just yet.
Is it potable?
I never risk it. Just ask. Every place I've been has always had signs posted for non-drinkable water. But it never hurts to ask. And as far as if you're out hiking; again, I rarely risk drinking directly from a river or stream without treating the water first. I just carry small tablets for purification purposes. Better safe than sorry. No telling when some dead animal might be up stream ya know?
So there you have it! The why's and why fors of water and travel. Now drink up and have a grand adventure!