PREPARE and bake cake mix in 13x9-in. baking pan as directed on package. Cool completely.
INVERT cake onto large platter; remove pan. Using a serrated knife, cut and scoop out a shallow rectangle from center of cake, leaving a 2-in. border of cake on all sides and a thin layer of cake on the bottom. Reserve removed cake for snacking.
STIR boiling water into gelatin in large bowl until completely dissolved. Stir in cold water. Refrigerate about 1-1/4 hours until slightly thickened (the consistency of unbeaten egg whites). (Or, see tip below for a quicker set method.) Pour thickened gelatin into center of cake. Refrigerate 3 hours or until set.
FROST borders of cake with whipped topping. Decorate with cookies, graham snacks and GUMMIES as desired to resemble a swimming pool. Store any leftover cake in refrigerator.
Fun Idea: Insert gummy fish and octopus candies into gelatin "pool" before refrigerating for an added treat!