Prepare cake batter and bake in 13x9-inch pan as directed on package. Cool completely. For easy removal of cake, line pan with foil before filling with batter.
Invert cake onto platter; remove pan. Use serrated knife to cut and scoop out shallow rectangle from center of cake, leaving 2-inch border of cake on all sides and thin layer on bottom. Discard or reserve removed cake for snacking or other use.
Add boiling water to gelatin mixes in large bowl; stir 2 min. until completely dissolved. Stir in cold water. Refrigerate 1-1/4 hours or until slightly thickened. Pour into center of cake. Refrigerate 3 hours or until gelatin is firm.
Frost borders of cake with COOL WHIP. Decorate with remaining ingredients as desired to resemble a swimming pool. Keep cake refrigerated until ready to serve.
OPTIONAL: Insert bite-size fish-shaped and octopus-shaped chewy fruit snacks into gelatin "pool" before refrigerating.