Preheat oven to 325°F (163°C).
Lightly grease a 9×13-inch casserole dish (avoid metal pans for best rice texture).
Place chicken breasts in the dish and season with salt and pepper.
Evenly spread uncooked rice over the chicken.
Sprinkle the onion soup mix on top.
In a bowl, mix cream of mushroom soup with water until smooth.
Pour the mixture evenly over the chicken and rice.
Cover tightly with foil and bake for 1 hour 15 minutes, or until the rice is tender and chicken is fully cooked.