Kokatat's Tributary Hydration System is an add-on pocket for the back of a PFD with a Hydrapak water reservoir and hose. The reservoir is BPA free, Phthalates free, and PVC free. If the hydration bladder were any bigger, you would notice the weight on your back, but the 1.5l bladder in the Kokatat Tributary is just right for half a day of paddling -- at lunch you can refill it. The Tributary comes with a zippered stash pocket on the side and reflective Kokatat logo so adding it to the back of your PFD also makes you safer at night.
The Tributary attaches to your PFD with "hook & loop" tabs that fasten to the shoulder adjustment sliders on the back and sides of most sea kayaking PFD's (not just the Kokatat brand pfd's).
It's great to get the water bottle off the kayak deck and not have a bottle rolling around between your legs or falling out in the event of a wet exit.