Wash and dry the watermelon, strawberries, and blueberries. Remove and discard the strawberry stems. Set the strawberries aside.
Using a chef's knife, carefully slice the top and bottom of the watermelon off. Slice the green rind off as well, trimming all the sides until you've achieved a square shape. Discard any trimmings, or save them for later use.
Pat the watermelon with paper towels to remove excess moisture. Place the sliced watermelon on a serving platter.
Using a long offset spatula, spread the thawed Cool Whip onto the top and sides of the watermelon. Spread the Cool Whip until the watermelon is evenly coated on all sides.
Place the watermelon cake in the refrigerator. Then make your strawberry roses: Take a sharp paring knife and carefully cut into one edge of the strawberry. Gently bend the "petal" away from the strawberry. Repeat this 4 times around the bottom of the strawberry. Then make another row of petals with the paring knife. When you get to the tip of the strawberry, slice the top piece in half.
Trim your wooden skewers to the appropriate size for the watermelon cake. Secure a strawberry on each skewer. Pierce the skewers into the places on the cake where you'd like the strawberries to be.
Garnish the top and bottom of the cake with fresh blueberries. Serve the cake immediately, or place it in the refrigerator.