The boneless fishes (skates, rays & sharks) all tend to retian Urea to maintain electrolyte balance. Soaking thin fillets in cold salt water a while helps leach that out.
BTW Beware of "round bodied" rays. It's kind of like grabbing an activated Taser when you shoot one. When we were diving off AL regularly there was generally some fool newbie about every other year who'd pop a "round flounder" with a borrowed pole spear. Sure can hear them a long way off when they do it if you're in the water with them.