The waterman is a surf backpack designed to carry food and liquid for "in water use" - while surfing, SUP riding or Kite surfing:
SUP riding - the custom designed 'chest plate' positions the backpack on the upper part of your back to keep it comfortable and securely in place, while remaining entirely non-restrictive for arm paddle motions. Its amazing, but the chestplate holds the backpack in position eliminating any annoying sway or movement you get from standard camelbak style backpacks.
Surfing - the Waterman's chest plate combines with the streamlined 'limpet' profile to secure the backpack with minimal drag especially while duck-diving, and is hardly noticeable when paddling while on a surfboard.
Kite surfing - the backpack sits high enough to stay out of the way of your harness.
The Waterman has been designed to be big enough for storing basic essentials without weighing you down - in fact the empty backpack and bladder combined weigh only 650 grams.
Apart from having hydration available to you at all times, the bonus of having the Waterman backpack with you at all times on the water is the practicality of carrying spare essentials such as food, sunscreen or surf socks - and of course there is also the safety element of being able to carry a mobile phone or gps. Not to mention protecting your keys or valuables by not having to hide them on the beach.
A note on the importance of remaining hydrated while SUP riding: New research has highlighted some hard hitting facts against the fallacy that "because you are on the water - you don't need to drink water". To the contrary, the Waterman is becoming increasingly recognized as a revolutionary new tool for providing the "one litre of liquid per one hour of exertion" requirement - especially relevant while you SUP and your full body is exposed to the sun.