PolarPro FTY-FTY Under Dome for GoPro Hero 7/Hero6/Hero5 - Black

My images have dark corners/ a dark vignette around the image. Is this normal? I previously had a different brand and didn’t have this problem.

Need it for hero 5 session ! Where can I get it ?

I'm looking to use this while snorkeling with Whale Sharks off Isla Mujeres, Mexico. Would this still be fully waterproof at 1-10 ft. depths?

will it work with the hero8