As there's nothing like trying it out one-self, I filled up the bathtub, pre-checked my DiveBox's waterproofness, put in the CamOne and gave both their first plunge together. For the record: with standard housing & factory-set focus, underwater footage is uselessly blurry indeed (screenshot from 1080p clip below to see for yourself).
Corrective action being required, I put on my reading glasses, got out a precision screwdriver, slightly unscrewed the two 1.5mm screws keeping the CamOne's optical element fixed in place and turned the lens counter-clockwise about 60°. Placed the camera snugly into the housing again and went for another dip. To my surprise that little twist already improved image quality quite noticeably, as in usable-if-need-be. Getting good enough focus, at least at close range, required several more twist-plunge-check cycles and, it being an overcast day, some more artificial light to reduce overall graininess.
In the end, I rotated the lens nearly 90° counter-clockwise* from the factory setting, i.e. a quarter turn, in order to obtain a relatively clear underwater image. Note that sharp below means blurry above!
So, definitely possible, but alas rather impractical on the go as this re-focusing involves fiddling with two tiny screws and requires some Fingerspitzengefühl. Whether in the middle of Yucatan's mosquito infested jungle after a sweaty hike, on a rocking salt sprayed boat before rolling back over into the sea, or, you get the picture, pretty anywhere near outdoor action, is not quite the right place nor time to open a housing and start screwing around with a camera. YMMV.
Meaning you have to decide beforehand whether to go for air or water footage, as shooting sharp in both mediums is not possible without giving the lens a spin. While acceptable for a dedicated above- or underwater cam, not being able to obtain mixed footage, not to mention one-take transitions, is IMO a very restrictive limitation.
