Preheat an oven to 350 degrees F (175 degrees C). Mix the 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 the chicken. Reduce heat to a simmer, and cook, stirring occasionally, until chicken is no longer pink in the center, 5 to 7 minutes. Drain the chicken; set aside. Whisk together the sour cream and cream of chicken soup in a large bowl. Stir in the chicken, then transfer the mixture to a 9x13-inch baking dish. Sprinkle the cracker mixture on top.
Bake in the preheated oven until bubbly, 25 to 30 minutes.