Waterproof Chart Case by Whetman Equipment.
Best case I've used! And I've used at least four different ones now.
 
These are in stock and I'll be ordering more.
 
From Whetman's site:
​Dry mapcase to stow on the exposed deck of a sea kayak and withstand being hammered by waves and the elements. Clips onto decklines using four micro karabiners with elastic shockcord.

Suitable to protect maps, documents, phones, passports etc.