And if you can't find one, which if you currently have customs will be a challenge, consider a cheaper OTS insert.
You will limited by the volume and tighness of the water shoe. You 'probably' need a very thin orthodic, of which the Superfeet, Gray, custom moldable is one of the thinnest. $70 +/-.
Take the water shoe and your current orthodic to someone who does this stuff, like REI, get whatever size best matches your arch, then confirm it will fit in the water shoe, and that the shoe has enough remaining enclosure to grip your foot in a raging torrent.
If you find a model that seems like it will work try inserting a (smaller, if necessary) out-of-the-package unit into the shoe and trying it on. You'll learn a lot at no cost. If it works, then mold. Your fitter can grind/cut away width to help fit the final product into the very tight-fitting water shoe.
This is all pretty straight-forward to someone who does it on a regular basis.
And it worked well for me on the Escalante.