Ad says "Film underwater up to 60 metres deep", but note this is the same housing supplied with the camera. Its problem is that the curved window acts as a diverging lens. If you're looking for the flat window case, as far as I can see (and it's confusing) you need Sony AKA-RD1 Replacement Door Pack for Action Cam which actually has 2 doors - quoting Sony:
1) Crisp diving images When you're shooting underwater, refraction and distortion are going to be an issue. To counteract this, the replacement door pack includes a special flat lens door that ensures clear and focused images to depths of 60 metres.
2) Clear underwater sound Filming a coral reef? Want to hear what all those fish have to say for themselves? The audio enhancing door for the waterproof case has a slight opening near the embedded microphone, enabling you to record clear, rich sound down to depths of 10 metres.
I'm guessing 1) is the 'square window' - may be sharper and wider, but give more 'fisheye' distortion ? I would have thought Auto-focus would mean the sharpness increase is small, perhaps traded for increased Chromatic Aberration ? In theory domed windows should be better, but both Sony + GoPro seem to have underwater focus problems which flat windows solve - it's not clear why !
2) is the 'round window', since it has 'stereo' written on it, with thin membranes opposite the mics.
... and before buying, check prices on sony's UK website !