In a large, deep skillet over medium-high heat, melt the butter. Stir in the onion and garlic, and cook 2 minutes. Mix in the coconut milk, water, rice, and chicken, and bring to a boil. Reduce heat to low, cover, and simmer 20 minutes, until rice is tender and chicken juices run clear.
Stir the spinach into the skillet, and cook just until heated through. Sprinkle with coconut, and serve.