Place chicken breasts in a slow cooker sprayed with cooking spray.
Sprinkle ranch seasoning over the chicken.
Mix the BBQ sauce and 2 tablespoons of the water in a bowl, then pour over the chicken. Reserve the remaining 2 tablespoons water for the cornstarch slurry.
Cook on high for 2–3 hours or low for 6–7 hours, until the thickest piece reaches 165°F.
Shred the chicken using two forks.
For thicker sauce, mix the cornstarch with the remaining 2 tablespoons water and stir into the crockpot. Cook uncovered until the sauce reaches the desired thickness.