You have already seen the GoPro-inspired diving masks with that special swivel arrangement on the mask top, haven't you?
Sure you've seen it. So have I.
But by the moment such masks were mass-produced, I have already made my own version. It was a simple clip with that action camera swivel mount added, and I tried using it for snorkeling. The next day I disposed of the thing, since the most footage the camera has recorded consisted of the surface-cutting. It was similar to those dramatic submarine movies moment when the periscope is lowered to crash-dive the boat.
So, no good. And then I started to think about the better place for mounting the camera. Mounting the camera to the snorkel mouthpiece was simple and easy, and it proved to be way better. It required the removal of the water-out valve, but no-one really needs this afterthought feature. It just adds more parts to go wrong, same as those anti-splash thingies on snorkel tops that are better removed. Such things are only there for mercantilistic reasons anyway.
What you get by mounting the action camera to the snorkel mouthpiece:
1. The camera is underwater the whole time, unless you want to record over the surface. Then you just raise your head.
2. The monitor is in your field of view, and you see what you are recording.
3. You can adapt the camera direction whenever and however you wish, start/stop it, watch the proper operation.
4. It does not snag on anything in the water, because it is where you can see it.
5. It does not impede your progress in any way.
The photos are all too self-explanatory, and you can see what you need to do. The only one more thing to get besides the snorkel and the CocaCola cap is one plastic 1/4"-20 screw to connect the GoPro-to-tripod-mount to the cap.
The cap is just to ensure that the mount accessory is properly oriented in the snorkel vent space, but if your snorkel is different, you could think up other ways to attach the mount. What I'm suggesting here is the alternative place for a camera - and there are many ways to achieve this. So let this short Instructable just fire up your imagination!
Enjoy the waters - and Dive Safe!