I have been 3D printing different mount designs for my GoPro in order to take photos of the seafloor from my boat / kayak for several months now. I am finally happy with the design and have posted it on Thingiverse if you want to print one yourself.
The mount connects a GoPro to a 1kg dive weight with additional cable ties. You can then lower the drop camera into the water to take photos and videos of the seafloor at different angles. I attach the mount to a diving reel with a stainless steel carabiner snap hook.
Here are some photos taken with the set up.
