I have the X-H1 which I use as my land camera. I love it, it is fantastic. It's not the absolute best camera at any one thing, but it is a great great camera at just about everything. (Studio, wildlife, street photography)
I thought about getting the Nauticam housing for it, to replace my LX-100, which is my underwater system. Long story short - I upgraded to the LX-100II, which fits in the same housing. I wound up adding the WWL-1 to my system.
The Fuji X-H1 could be brilliant underwater, but it depends on what you want. For video, the image stabilization would be really helpful. However, I don't know how well the X-H1 will execute a custom white balance underwater, which is really important for uw video work. Canon and Panasonic seem to be best at this. Alternatively, you can compensate by adding a filter (Magic Filter or Keldan red filter) to the camera lens, though this then pose a challenge switching between stills and video. The question of white balance matters less for stills because you are shooting to your strobe(s), and can easily adjust the white balance on raw images. (It is much harder to adjust white balance on video files)
There are no native autofocus fisheye options from Fuji, though there are 2 native wide angle zooms (10-24 F4 and 8-16 F2.8), and several wide primes (12mm, 14mm, 16mm). There are many more lens options in the micro four thirds ecosystem.
For me, it came down to cost and size issues. I upgraded my camera without having to replace the housing, which was awesome. My kit fits in my carry-on bags (just barely), which is also really helpful, and that alone keeps me from upgrading to a larger sensor system. The image quality is better with a larger sensor: the LX100 II uses a slightly cropped micro 4/3 sensor, vs APS-C for the X-H1, but the quality gain just was not enough to justify the increased cost and size. There is no wrong answer, it's just a matter of what works best for you.