Line 13x9-inch pan with foil; spray with cooking spray. Prepare cake batter and bake in prepared pan as directed on package. Cool completely.
Invert cake onto platter; remove pan and foil. Use serrated knife to cut 1/2-inch-wide strip off each side of cake.
Cut each strip lengthwise in half; set aside.
Spread trimmed cake with 1-1/2 cups COOL WHIP. Stack cake strips around outside edges of cake to make rim; frost rim with remaining COOL WHIP. Refrigerate until ready to use.
Add boiling water to gelatin mix in large bowl; stir 2 min. until completely dissolved.
Add ice; stir until gelatin begins to thicken.
Remove any unmelted ice.
Pour gelatin into center of cake. Refrigerate 4 hours or until gelatin is firm. Decorate with remaining ingredients to resemble an aquarium as shown in photo.