In a large saucepan over medium heat, cook ground beef and onion until meat is evenly brown; drain excess fat. Mix in water, tomatoes, kidney beans, tomato sauce, and taco seasoning mix. Cover, and simmer for 15 minutes. Remove from heat, and stir in the avocado.
Ladle hot soup into serving bowls. Pass cheese, tortilla chips, and sour cream to top each serving.