Comparing kapalbhati pranayama just with any work out, if you are exerting your abdominal muscles to the extent that you are sweating, you can sip warm or normal water depending upon the weather. I think the important thing you should take care of is not to gulp a cup of water (8-10 ounces) and the water should not be cold. Else you won't be able to do kapalbhati effectively. It is the abdominal muscles and the abdomen which play role during kapalbhati. The water straight way reaches to the abdomen within the intestines. Filled in intestines will not allow you to do kapalbhati for a long time as it may cause pain or discomfort. So, it is better to drink water like sipping a cup of tea so as to prevent dehydration. Slowly intake of water is easily and quickly absorbed into the body from the intestines. Because sipped water is mixed with the saliva (alkaline), it can help you to counter the acidity in the stomach.
You should not take other fluids which require digestion. As well, you should always do kapalbhati on empty bowels in the morning for good results. In the evening, there should be after atleast 4 hours of normal meal (not heavy meal) and that too when you are relaxed.
