I did something similar with snake skin. I cut the blank for a pouch sheath from 4-5oz veg tanned leather and glued a piece of the snake skin to it with a latex base glue, which was chosen because of it's flexability. After the glue cured overnigh, I trimmed to snake skin to the leather blank and then sewed on the belt loop and glued the welt as one would normally do. After it was all glued together, I laced the edge of the pouch. If I was going to sew the pouch I would have cut a frame out of 1-2oz leather and glued that over the snake skin leather before assembling the sheath and sewed through it. That makes a better looking sheath and would help prevent the thin decorative leather from pealing away at the edge. As to you question about whether a fish skin leather is water proof-no. You might want to treat the inside of the sheath with a conditioner just as a precaution against any chemical used to tan the ray skin leather from leaching through and effecting the knife blade. It is also best not to store a knife blade in a sheath.
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Above is the snake skin covered sheath that I made.
Doug Lester