Gopro mounted on nose of sup is there any way of turning it off & on between waves apart from bending down or having it on video all the time

You can use the wireless option. Newer GoPros support it, older models need the WiFi backpak. And you need the controller on your wrist. I tried it a few times but found it would lose its connection frequently - I lost confidence that it was actually recording.

I ended up buying the battery backpack so I could leave it in video mode the entire time. It will last for well over three hours.