The Laowa 24mm macro probe is amphibious up to the USB socket. It has a flat port which I suspect is why it is less sharp away from the optical axis. It does not seem all that sharp at infinity possibly due to the small aperture range (I have this lens). I am interested in hearing about the mirror gadget, which you have, before forking over the money for one....
f/2.8 is the MAXIMUM aperture of the Nikon and Canon macro 105 or 100mm lenses. It is interesting that the aperture of the prime lens is conserved. I have a vintage fisheye conversion lens made by Kenko (bought it in the 70's) that has an effective aperture that is dependent on the focal length of the prime lens. I bought it with a Bay50 adapter to use on the Hasselblad 80mm lens - I recall the effective max aperture with that as being rather slow like f/11 (have not used it in decades).
I have a shot done with the Laowa lens on my site, found two actually:
NOT a salmonid so in the different place on my site. Note that this species is also found in Europe.
These shots were taken right at the edge of a lake, in just a couple of cm of water. Would the mirror help with the angle? I was positioned on land so had to point the rig at a downward angle which limited me quite a bit.
PS - just looked at the exif data - it is just one shot - the one looking downward - file with Z6 in the name.