I checked with him and he doesn't mind if I share his design.
This instructable assumes that you already have a helmet and a waterproof camera, so it's mostly about how to mount the camera on your helmet.
And the idea should work for anyone who likes to get out on the water and doesn't mind violating some sort of dress code of their sport. I'm thinking of surfers for instance and kite boarders and sail boarders, none of whom seem to wear helmets.
Step 1: Materials You Will Need
a foam block,
some bunji cord
and three cable ties, or lacking those, string or wire.
Step 2: Carve the Foam Block and Attach It to the Helmet and Attach the Camera to the Foam Block
The foam block not only cushions the camera, but it also sets the camera at the appropriate angle. The idea is that the camera will be pointed at whatever you're looking at with your eyes straight ahead. You probably have to give this a few tries to get the angle of the camera right.
Attach the foam block to the helmet with the cable ties.
Attach the camera to the foam block with some light weight bunji cord.
Install another cable tie to the helmet to attach the camera strap to in case the camera works its way loose of the bunjis.
Step 3: Shoot Video
If you want to see what the video looks like that Mark was shooting, go here:
The attached picture shows Mark going out through surf at San Simeon. That's William Randolph Hearst's pier in the background. He built that to bring in materials to build the Hearst Castle. Hearst is dead so we can't ask him to do an Instructable on how to build a castle, but if you want to get the Hollywood version of the Hearst story, rent Citizen Cane with Orson Welles as Hearst.