a "good" helmet cam (I assume you mean action cam that is not just specific to helmets) is often touted as limiting for kayaking if used in the conventional way: shooting subjects that are relatively far away from the camera.
Most helmet cams are intended as point of view where the subject is about max 10 feet away or it suddenly becomes too small to see.
Kayakers in a group will appear very distant unless they are very close to the photographer.
In saying that some helmet cams offer narrower field of view but I find that operating them without a viewing screen (something that is not standard on most cams) is a hit and miss; there is a reason that most helmet cams use wide angles: no need to focus and no need for viewfinder (cheaper and more compact to manufacture).
If however you understand the limitations that most helmet cams have and embrace an aggressive “in your face” style of filming than helmet cams are excellent for water sports as they offer incredible waterproofness and ruggedness.
Sample of “aggressive” style of filming with cam close to the subject: http://youtu.be/6oT2uUJADg8
A “good” helmet cam is the very popular GoPro; there are others but GoPro seems to have more mounting options for kayaking and is seriously waterproof (not just water resistant) as salt water kills anything that is not super solid.
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