Preheat the oven to 350 degrees F. Grease a 9x13-inch baking dish.

Make the brownies: Stir brownie mix, oil, eggs, and water together in a medium bowl until well blended. Pour batter into the prepared pan.

Bake in the preheated oven for 15 minutes.

While the brownies are baking, make the topping: Break graham crackers into 1-inch pieces and place into a bowl. Add marshmallows and chopped chocolate and toss to combine.

Remove brownies from the oven and sprinkle with topping ingredients. Return to the oven and continue to bake until a toothpick inserted 2 inches from the side of the pan comes out clean, 7 to 10 more minutes.