I'm in the underwater photography niche (currently with an R5) and would like, in no particular order:
- A global shutter body with resolution comparable to the R5 that can flash sync at speeds above 1/200 or 1/250. I know it's not coming this year, but at least commit to working on one. Flash sync is a massive deal underwater since strobes are required for most shots. Not being able to use faster shutter speeds constrains exposure options quite a bit, especially when it comes to the background (getting it nice and dark for example).
- An RF fisheye lens to replace the old EF 8-15 which is getting difficult to find and requires the RF adapter which adds unneeded weight and length.
- An L series RF 1:1 macro lens in the 80-85mm range that's not the 85mm f/2 STM (with its 1:2 magnification, slooow AF, etc...). I have the RF 100 macro and it's great, but it would be nice to have an equivalent quality option at a slightly wider focal length for when I'm not after the tiniest of subjects.
Great to have someone else in the group doing underwater with the R5

- I would suggest that current global shutter tech would not be great for us ie A9iii with lower resolution and lower dynamic range. I haven't found that rolling shutter is an issue underwater. The A9iii is much more expensive than the R5 so I don't think that high res/high dynamic range/cheaper cost will be possible in the R5ii.
Flash sync <1/250s would be nice. Neither Sony nor Nikon have provided a solution though so perhaps Canon can surprise us. More likely in the R1 though.
I'm using DS232 strobes and even they have problems keeping up at 10fps especially with higher outputs. Using TTL is much faster for composition rather than using manual strobe power settings but I appreciate the skill if you do that.
Higher strobe output to overpower ambient light is nice but keeping accurate distance-to-subject with blowing out the highlights is a challlenge for me.
I am doing a lot of light shaping in post. Always better to do it in camera but having the flexibility in post seems to be working well for me.
- My EF8-15/4 is still working find but I am not following your comment on length as the RF8-15/4 is still the shortest port length vs the other wide angles and within 200gm of the alternatives...
EF8-15/4 = 20mm port extension 700gm (including adaptor)
RF10-20/4 = 28mm 570gm
RF14-35/4 = 50mm 540gm
RF15-35/2.8 = 78mm 835gm
Personally, I am planning to upgrade to RF14-35/4 later this year which will reduce my use of my fisheye even further as it is much wider. I will still use it for astro though. The 'fisheye' look is still unique though but I find it hard to have good compositions.
- Depending on the vis, I have often taken shots of bigger subjects with my 100mm macro. I always enable full focus range just in case of bigger / more distant subjects. If it hunts then I use my hand to bring it back to close focus.