Purchase metallic gray pants, tights or sweatpants according to personal preference. Trousers or tight-fitting pants will be most convincing since they are closest to what Sharkboy wears in the film.
Buy a black turtleneck to be used as the layer underneath Sharkboy's shield vest.
Use a gray or metallic silver utility vest in place of Sharkboy's shield. You can find such a vest at a sports shop or camping supply store.
Draw shark teeth in the shape of a shark's open mouth on the front of the vest using a white fabric marker to emulate the design of Sharkboy's shield.
Cover thin sheets of cardboard with metallic spray paint. Allow the cardboard to dry overnight.
Trace the shape of a shark's fin using a pencil and then cut out the shape using a box cutter. The size of the fin will vary according to the height of the person wearing the costume; measure the fin to reach from the base of the neck to the middle of the back.
Attach the fin to the back of the utility vest using a hot glue gun. The fin should stand up on its own once it dries.
Finish the costume with a pair of black elbow length gloves.