I use a 14-42ii in a macro port 35 on a Panasonic GH5 with the AOI UWL-09F and a INON UCL-165 mounted on the arms whenever I dive at a new site, unless I know I'm going after something very specific.
However, the need to burp the wet lens upon entry can be chore, and very cumbersome with gloves in cold water. In low visibility water I occasionally get particles between the macro port and wet lens rear optic, which can be difficult to clear and incredibly frustrating. Less of a problem if you dive in warm clear water.
If I get the chance to do subsequent dives at the same site I'll switch to my fisheye lens in mini dome or 60 mm macro lens with 35 mm macro port and 30 mm mini extension.
That's just covering the practicalities. In terms of corner sharpness, you're comparing apples to oranges.
A UWL-09 is a conversion lens and gives 130 degree FOV at 14mm. I usually go no lower than f5.6-f8 to retain sharp corners.
A fisheye lens will give you 180 degree FOV at 8mm. I use f4 when shooting CFWA, I like to reduce DOF and blur the corners a little to get the subject to pop. Just my preference. I use f8-f11 when shooting scenic wide angle to get everything in focus. You'll probably get better IQ with a dedicated fisheye lens.
Rectilinear wide angle is a tricky subject. You will need a wider dome than a fisheye lens and need to stop down significantly to give enough depth of field to get sharp corners. The bigger dome will cost more but will be easier for splits, if and when you do them. Check out this link: https://interceptor121.com/2019/10/13/using-rectilinear-wide-lenses-underwater/
If your budget allows. I'd go 14-42 with AOI-UWL-09 first then a fisheye in dome port. I've only shot splits once, and that was an evening dive and I borrowed an 8-18 lens and 7mm done (and whole camera rig, tbf) from someone on the boat as they didn't want to get wet again. I'm currently not shopping for a rectilinear wide angle lens.