Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split like that.
Fill nylon stocking with concrete for the base. Create gills underneath the cap with thin sheets of metal. Sculpt a mold for the cap, and press it into WET sand lined with a thin sheet of plastic, and slowly fill with concrete. Add realism by removing small wedges of concrete along the edge of the cap. Cap mold: A circle of cloth sheeting could be draped over a bowl/balloon, and brushed with concrete. The wavy edges could be created with rolled up newspaper under the edge of the cloth.