Place chicken breasts in slow cooker. Pour in water and sprinkle chicken with taco and ranch seasonings. Cover and cook on high for 3-4 hours or on low for 6-8 hours.
Remove chicken to a cutting board and shred between two forks; return to slow cooker and mix well.
When ready to serve, fill tortillas with shredded chicken, drizzle with ranch dressing, and top with slaw, red onions, cilantro, and avocado. Serve immediately.