Combine chicken pieces and water in a pot and boil until the water has evaporated, 5 to 10 minutes. Remove chicken pieces from the pot.

Heat oil in the same pot and cook onion, ginger, and garlic until softened and fragrant, about 3 minutes. Add chicken pieces and tomatoes and cook for about 5 minutes. Stir in coconut meat, coconut water, and bok choy. Simmer until chicken is cooked through, about 15 minutes. Season with fish sauce and salt.