Cough syrup +/-water. It is best to stay hydrated when taking an expectorant cough medication. I do not recommend cough medication to my patients. I do however prescribe guaifenesin-containing medications. When i do, i ask that they take the medication with a glass of water. Ask your pharmacist about the best way to take the specific medication prescribed for you.
EITHER IS FINE. You will not decrease efficacy one way or the other with normal amounts of water.