Season the chicken breasts with salt, black pepper, and dried Italian seasoning on both sides.
In a skillet, heat olive oil over medium heat. Add the chopped onion and minced garlic and sauté for 3-4 minutes until softened and fragrant.
Place the seasoned chicken breasts in the bottom of the slow cooker.
Pour the sautéed onion and garlic evenly over the chicken.
Add the marinara sauce and water to the slow cooker, stirring gently to combine with the onions and garlic.
Cover and cook on low for 3 hours until the chicken is cooked through and tender.
Remove the chicken breasts from the slow cooker and shred them using two forks.
Return the shredded chicken to the slow cooker and stir to combine with the sauce.
Add the uncooked ziti pasta to the slow cooker and stir to submerge the pasta in the sauce.
Cover and cook on low for an additional 1 hour and 30 minutes, or until the pasta is tender, stirring halfway through to prevent sticking.
In a small bowl, combine the ricotta cheese and half of the mozzarella cheese.
Once the pasta is cooked, dollop the ricotta and mozzarella mixture evenly over the top of the ziti in the slow cooker.
Sprinkle the remaining mozzarella and grated Parmesan cheese on top.
Cover and cook on low for 10-15 minutes until the cheese is melted and bubbly.
Sprinkle chopped fresh basil over the top before serving.