It's obviously fake but a very clever montage. If you look at it for a while. The symmetry between the canoe and the shark is uncanny; including the proportions of the canoe vs the shark, the angle of the paddle compared to the pectoral fins, the dorsal fin vs the mans head. The bubbles from the shark imply speed but the shark does not appear to be chasing the canoe, just following it. The canoeist seems aware of the shark from the angle of his head but he is also calm. The distances also make it a very powerful image as the canoe is much further from land than the shark is from the canoe, so you know the canoeist is on his own. Once you get over the initial shock, to me it's actually quite a peaceful image as the power is all with the shark (nature). Man is enchroaching on his realm and calmly being driven back to land.
http://www.whitesharktrust.org/pages/mediaarticle/media27.html
http://snopes.com/photos/animals/sharkkayak.asp