Cut out circles from dark blue crafts foam. Reserve a section of the boa for a tail (about 1-1/2 to 2 feet). Cut the rest of the boa into small tufts. Hot-glue the tufts to the crafts-foam circles. (Reserve approximately eight to 10 tufts for the swim cap.) Hot-glue the tufted circles to the sweatshirt, sweatpants, and blue gift bag.
Have child stand with arms out at shoulder height, parallel to the floor. Measure arm length and distance from armpit to waist. Cut two pieces of blue gauzy material into rectangles with the same dimensions as the child's measurements. Hot-glue or stitch a piece to the underside of each sweatshirt arm and the outer edge of the sweatshirt body. Round the edge of each rectangle piece and cut out triangle shapes along the edge to form "spikes."
Spray-paint the swim cap blue; let dry. Glue reserved boa tufts to the swim cap. Glue or stitch the reserved boa tail piece to the seat of the sweatpants. Wear costume with swim mask and fins for trick-or-treating.