Lightly coat the inside of a 6-quart oval slow cooker with cooking spray. In a large bowl whisk together water, sour cream, and eggs. Add cake mix, flour, and sugar and stir just until combined. Stir in marshmallows. Spoon into prepared cooker. Cover and cook on low for 2 1/2 to 3 1/2 hours or until an instant-read thermometer insterted in cake registers 180 degrees F (center may still appear moist), rotating the crockery liner once halfway through cooking.
Drop spoonfuls of marshmallow creme over hot cake (it will spread to an even layer as it stands). Sprinkle with chocolate pieces. Remove crockery liner from cooker. Let stand, covered, for 1 hour. Just before serving top with graham crackers.