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Several years ago I swam up a few rivers in Kakadu Australia with my Hasselblad 501c in one of these dry bags :
http://cascadedesigns.com/en/sealline/dry-bags/see-bag/product
Make sure to use one of these thick vinyl ones - not the lighter weight proofed nylon bags as they are easily pin pricked.
Absolutely agree. Those bags are absolutely water proof. Put your camera in there together with your dry clothes and secure the bag with a rope to the Canoe. Only take photos at land or when the water is very calm. When the water is rough, every hand is needed so you won't have time to take photos.
Edit: The nice thing about this bags is, that there is enough air in them so they float.