Oh, my head! My head hurts from hearing my mother say over and over and over again,"You need to give that baby some water!" and me telling her over and over again,"No, I don't. He nurses/drinks milk so he gets plenty of water." This went on from the day he was born and was finally put to rest a month ago at his 12-month checkup. I asked my pediatrician if I need to give my toddler water, and she said,"No. He gets plenty of water from breastmilk or milk (formula has plenty of water, too). It is not necessary. And, no, his kidneys will not suffer from lack of water. (Yes, my mom actually believes theat.) He can drink up to a few ounces of water a day, but no more. Since your son needs the calories, just forgo the water and feed him only milk (to drink)."
So, there's your answer! Cross you fingers that my dear mother will stop pestering me about it too!
Also, from what I've read, giving too much water to a baby can cause some dangerous problems (i.e., water intoxication) and will fill up the baby, thus reducing the vital calories a baby needs everyday. If I were you, I'd avoid that route.
Finally, you may want to wait until your little one is 6 months old before giving the rice cereal to help his digestive system mature before feeding him solids. I know a mother who fed her son solids early and he developed some serious food allergies. She waited until 6 months with her second, and that baby developed no food allergies.