In Victorian times they used to use lead cofins.
As for getting wet, a couple of coats of high gloss emulsion should do the trick.
Just wait, at some point in the future Krispy Creme will adapt the machine they use to glaze their donuts. If you replaced the donut glaze with a polymer coating that hardens on exposure to UV you could coat one side of the corpse, harden, flip, harden the other side.
If you made the plastic opaque you could then have a CAD machine injet a likeness of the deceased onto the outside of their shell, or alternatively leave blank for a more classical look.
Best of all it will be possible to keep your deceased loved ones with you in your house, forming a decorative sculpture that is sure to be a talking point for every visitor.