PREPARE cake batter and bake in 13x9-inch pan as directed on package. Cool completely.
REMOVE cake from pan; place on platter. Use serrated knife to cut and scoop out a shallow rectangle from centre of cake, leaving a 2-inch border on all sides and thin layer of cake on bottom. Reserve removed cake for snacking or other use.
ADD boiling water to jelly powders in large bowl; stir 2 min. until completely dissolved. Add ice; stir until jelly begins to thicken. Remove any unmelted ice. Pour jelly into centre of cake. Refrigerate 4 hours or until jelly is firm.
FROST borders of cake with Cool Whip. Decorate with remaining ingredients as desired to resemble a swimming pool. Keep refrigerated.
serving size = 1/20th recipe (68 g)