Place chicken in a single layer in the basin of a slow cooker. Season with salt and pepper and add water. Cover and cook 2-3 hours on high or 4-5 hours on low.Remove to a plate and cover with foil. Rest 5-10 minutes.
It's important to only cook meat in a single layer in the slow cooker. You may reduce or increase the number of chicken breasts you use accordingly, but make sure they are all in contact with the bottom of the slow cooker.