If you need a proper hydration bag, you should get a Camelbak. However with a little bit of innovation, you could simply turn any messenger bag into a hydration bag. All you have to do is get yourself a bladder/reservoir and slip it into your bag. I myself bought Hoser, a bladder by Platypus and stuff it into my small Cocotte messenger bag. Just make sure the tube is reachable so that you can bite the valve with ease anytime while riding. To have your water on the go is one of the great benefits of hydration pack. This allow you to focus your eyes on the road/trail and enjoy your water at the same time.