Both the Nikkor 16-35mm f/4G ED VR and the Nikkor 8-15mm f/3.5-4.5E ED lenses are awesome.
Honestly, it’s a matter of personal preference. Both have distortion, but it’s a different type of distortion. Above water, a rectilinear distortion feels a bit more natural and therefore the lens is much more useful above water.
However, when shooting underwater, since there aren’t many straight lines, fisheye distortion actually looks more natural so many would prefer that.
If you ask Ran, Mozaik’s in-house UW Photo Pro, between these two lenses. He would get the 8-15mm since he is a huge fisheye fan!
Fisheye lenses can also be used with a smaller dome, if you ever want to travel lighter, you can get a mini-dome for it and get excellent CFWA shots (Close Focus Wide Angle) which fisheye lenses are famous for.