This works for the Kodak playsport type cameras only. I will try and put a link to the vid I made about this at the end and hope it works. Basically you take a block of foam and cut a slot into it so you can wedge the camera a few cms into the foam. Attach the cam. lanyard to a buckle strap passed thru the helmet vent so the camera if knocked off with the foam is not lost in the water. Mount the foam block on velcro rails stuck to the top of the helmet, and off you go. I have used this system with my playsport for 2plus years on the water and sledding on snow without any untoward incident whatsoever.
helmet cam mount - YouTube