From http://www.helmets.org/ this is what they say about Kayak helmets:

Whitewater sports have their own helmets. The impacts are lower than in bicycling, but more frequent, and the sharp rock hazard results in a need for more coverage. Water must drain from the helmet as well to prevent "bucketing," and some canoeists mention the ear as a vulnerable area if your head is being dragged under water. A bike helmet will be better than a bare head for whitewater, but a canoeing helmet should be considerably better adapted to the sport. ASTM is working on a whitewater standard, but does not have it in place yet, so you are on your own to make a choice. There are some whitewater helmets out there with multi-impact EPP foam in them instead of the squishy stuff, which should be an improvement. (As an illustration, Cascade is one brand made that way.).