What a fantastic mod! I love the magnet and the reed switch.
Connect this one to the contacts of the record button and mount it on the back of the case, right under the screen. Now, waving a magnet by the bottom of the screen will start and stop recording underwater, without breaking the seal of the case.
I’ve often thought of building an underwater case for a camera but doing the interfaces for the buttons has always been a daunting task. For a while I was thinking I would use a water proof IR remote but never worked quite right. Plus it was a pain to mess with underwater. The magnetic switches are perfect. I wish I would have had this while I was on the beach…