I have one that I use a lot on day trips (the only kind I seem to be doing anymore). I used to always use US Army pattern plastic canteens (water bottles) but I realized I never drank that much water, irregardless of how much I was supposed to be drinking.
Before that I used those imported water bottles, the clear ones with the red stoppers and green caps, Swiss probably, but those have disappeared from stores around here. There are all sorts of other plastic bottles in all shapes that people presumably use. However, since you now see people carrying water bottles all over the places and using the rather flimsy plastic bottles that bottled water comes in, I figured that an ordinary soda bottles would do just fine and they do. They even fit certain pouches on some of my packs better than anything else.
In theory, the caps are easy to lose but that doesn't seem to be a serious problem. The little loop on a US Army plastic canteen is just right for holding a candle, however.
One thing I've been wondering is if they really are pilot's water bottles.