Drinking water with meals dilutes the gastric juices and the natural enzymes. So if you have trouble digesting food ,sip water if you have to. Also remember water fills you up and you might end up eating less food when you are trying to gain weight. You do not have to adhere to this rule as if your life depends on it, but space it according to your personal requirements. Reasoning for proper food combining.
Protein foods require an acid environment for proper digestion.
Carbohydrates require an alkaline environment. Carbohydrates are starch and sugars. Avoid eating carbohydrates and proteins. Dr. Herbert M Shelton. Food Combining Made Easy, Superior Nutrition , and Volume Two of the Hygiene System on Foods and Feeding.