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 over the chicken.
Add the marinara sauce and water to the slow cooker, spreading evenly over the chicken.
Cover and cook on low for 4 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 well to combine, ensuring the pasta is submerged in the sauce.
Cover and cook on high for 30 minutes, or until the pasta is tender, stirring halfway through cooking to prevent sticking.
Once the pasta is cooked, sprinkle the shredded mozzarella cheese and grated Parmesan cheese evenly over the top.
Cover and cook on low for an additional 10 minutes until the cheese is melted and bubbly.
Stir in the chopped fresh basil just before serving.
Serve hot and enjoy your crockpot chicken ziti!