Bring a large pot of lightly salted water to a boil. Place spaghetti in the pot, and cook 4 minutes. Mix in the broccoli florets. Continue cooking 4 to 6 minutes, until spaghetti is al dente. Drain, and transfer to a large bowl.
Melt the margarine in a skillet over medium heat, and cook the chicken and onion 5 minutes, until chicken juices run clear and onions are tender.
In a bowl, whisk together the soup, milk, water, and cream cheese until smooth. Stir into the skillet with the chicken and onion, and bring to a boil. Reduce heat to low, and simmer 5 minutes, or until slightly thickened. Toss in the bowl with the spaghetti and broccoli to serve.