I tried a bike-mounted Camelbak for about five minutes before scrapping it. Every time you get on and off the bike, you have have to fuss with the tube, and if you forget about it, you get clotheslined. Bottles and cages work a lot better. Unless it's extraordinarily hot or desolate, two large bottles is fine. If I need extra water, I either lash a Platypus (takes up almost zero space when empty) to my rack, or buy a couple 20 oz Gatorade bottles and distribute them in the front panniers. Some people use the cage under the downtube. I've never bothered, but if you do the trick is to either use a bottle with a covered spout, or unscrew and pour from the mouth of the bottle into a clean one. It gets pretty gross down there.