The Westwater backpack is a monster waterproof backpack, offering submersible protection for several days worth of gear. These bags stack nicely in rafts, and the removable backpack straps and waist belt help to haul your gear up the beach to the campsite.
Removable, padded shoulder straps and waist belt
Durable top lug handle
Currently available in clear color only.
 
On the spectrum of dry backpacks, with one end being a dry bag with shoulder straps and the other being an ultralite backpack with slightly more waterproof material, this is more toward the dry bag with straps.  It is more backpackable than that (not super heavy material and has a hip belt) but if you're looking for something with more structure/back support and your adventure is unlikely to involve full-submersion of your pack then you might want to consider the Hyperlite Southwest.