Hello Jim:
Basically the author uses Reynolds "Release" brand of aluminum foil, which is not supposed to stick to the casting medium, which is "Enviro-Tex Lite".
Crumple up the foil, spread it out on something flat (the author uses a shallow pan to catch the casting material), add a couple of drops of resin dye to the Enviro-Tex to make the 'sea' colour you want, and pour over the aluminum to fill the shallow pan.
After it sets, turn it over onto a piece of acrylic sheet (to support the 'water', and peel away the foil.
Bob Santos gives more details in his article, but that's it in a nutshell.
Regards,
Bruce