The Zeiss 16/70 is a great travel lens and although a little bulkier, it is far better optically than the 16/50 PZ. My family and I have used both lenses extensively with A6000 and A6500.
The Z 16/70 works fine behind a 7" dome. We use a Zen glass dome. It doesn't require a diopter and the corners are OK with the usual caveat of the need to stop down when using a dome (at least f8 to f11 depending on your personal sharpness standards).
The 16/70 range underwater on an APS-C camera is a bit of a Jack of all trades but master of none. As a rectilinear lens, at 16mm underwater, it is not very wide but still fine for reefscapes etc. although a fisheye lens would allow you to get a lot closer and provide more dramatic perspectives. This focal length range does work well for medium to large size fish portraiture and for catching the more shy large pelagics. The greatest drawback of this lens underwater is that it doesn't focus close. Unlike say the Sigma 17/70 macro on a cropped sensor DSLR, this lens just doesn't cut it for macro or anything small, so full utilisation of the versatility of the zoom range is negated.
Although I haven't tried it, the Nauticam WWL-1 wide angle conversion lens is said to work very well in front of the kit 16/50 PZ and flat port and provides full zoom through and up to 130 degrees angle of view. The sharpness is apparently remarkably good despite the limited optical quality of the kit lens.