My favorite bag is the Lowepro Versapack (my reviews here and here). The only thing missing about that bag are pockets: there's nowhere to stash a bottle, a wet umbrella, or even a speedlite (just in case you need easy access for one).
This was solved by the Lowepro Photo Hatchback which has stretchy side pockets for your beverage or even a GorillaPod (my review the Photo Hatchback here) while my review of the GorillaPod is here. But I still want to use the Versapack. So what's the solution?
The Lowepro S&F Bottle Pouch.
It's made up of a stretchable neoprene cloth and it can carry a 16-oz bottle (a "Tall" size drink if you buy your drink from Starbucks). You can attach it to any Lowepro bag that has the "Sliplock" attachment tab or its equivalent. You can even attach it to your belt!
So, next time you want to have free hands when you do your photography and you still want to keep your water bottle at hand, then I suggest you get the Lowepro S&F Bottle Pouch.
So get yours here:
Gear used to make this post: