Preheat the oven to 350 degrees F (175 degrees C). Combine crackers and melted butter together in a bowl; set aside.
Bring water and chicken stock to a boil in a pot over high heat; season with salt and black pepper. Add chicken; reduce heat to a simmer, and cook, stirring occasionally, until chicken is no longer pink in the center, 5 to 7 minutes. Drain and set aside.
Whisk cream of chicken soup and sour cream together in a large bowl. Stir in cooked chicken.
Transfer mixture into a 9x13-inch baking dish. Sprinkle cracker mixture on top.
Bake in the preheated oven until bubbly, 25 to 30 minutes.
