Heat oven to 350°F. Line 13"x9" baking pan with foil; grease.
In food processor, pulse crackers and sugar; add butter. Press into pan.
Bake 8 min.
Mix brownie mix, oil, eggs and 3 Tbs. water. Spread over crust.
Bake until set, 15-20 min.
Arrange marshmallows over brownie.
Broil about 10 sec.