There is still also a niche market for underwater photographers. Housings are available for some smartphones now but the photos are mostly crap due to small sensors and poor integration with external strobes.
Nope. Go Pro still images look terrible once you go deeper and lose a bit of natural light. They don't really work with external strobes either. But for video they do fine as long as you have good lights.
The preferred hobby tier underwater still camera is now the Olympus TG-6 with a cheap plastic housing.
Sorry but no - if by underwater you mean diving, and not putting your hand 0.5m underwater or similar depth snorkeling. Gopro has tiny sensor and crappy lens and it shows, even if you set the bar of quality as low as phone screen-only.
You can end up with reasonably nice pictures from time to time, I've ended up with dozens from all activities I've mentioned (and that's using gopro hero 2 with custom housing in the past, now newer variants), but you will lose many more moments due to rather poor device performance.
The key word is niche. The area that these companies can ONLY afford to cater to. The Canon Rebel users of yore, taking slightly out-of-focus holiday snaps are totally lost to phones.
