I think I have to agree with Tim. It's an expensive hammer looking for a small nail.
That said, it could be useful for large animals, Tonga Humpbacks come to mind. Also sharks, dolphins, baitballs.
The small footprint would be great for freediving. That is where the 13mm also excels, compared to a domeport. However the above applications are typically "Blue water" shots where there isn't much in the corners anyway...
I have the 15mm, 20mm and 28mm wet lenses from the Nikonos V days, which work beautifully on the Sony A7 platform. The 15 and 20 are amazing optics. Can't say I have ever chosen the 28mm though. Maybe I will have to try it next trip as a testbed to see if the RS version is worth the effort. (Of course, focus becomes a bigger issue with longer lenses, so this may be an unfair comparison compared to the autofocus 28mm RS. With the 15mm using zone focus, everything is sharp from infinity down to about a foot and a half.)
It is interesting that the lowly, low priced 28mm R-UW seems pretty much to have disappeared off of the usual sites, and I am guessing they will re-appear at a higher pricepoint:((
ian