Go pro photo quality?

I currently shoot with a Nikon d40 and a Sony a330. I've been wanting to do some more underwater photography looked into getting a housing for either of them but it's pretty price so recently I've been looking at the option of getting a go pro. But I'm unsure of the print quality and the ability to blow up the images. I'd probably use the video but not as often as the stills. I've also read that you can pull stills from videos and they are relatively good quality. I've been looking at reviews of the go pro hero 4 silver over the black mainly because of the lcd display and I don't think I need to take 4k videos. I'm looking for honest opinions and or other people's experiences with printing or using go pro stills.

When I want to shoot underwater, I use the less than $90 Nikon Coolpix S31. It is waterproof, dust/sand proof and drop proof. It also has a 3x optical zoom (GoPro's have a fixed wide angle lens. No zoom) and can shoot both excellent stills and Full HD video

For stills, the GoPro is pretty bad. It's first and foremost a video camera. The sensor is way too small with far too few pixels. Although the lens is extremely wide angle which is ideal for underwater shots of corals and fish, the lens does produce a great deal of wide-angle distortion. Underwater the distortion isn't noticeable because you're not dealing with anything that has straight lines. Above water, and the distortion becomes very apparent.