They are watching the output of Seth's system. His volume is too high. The function of the Large Intestine was to absorb water, now that he doesn't have one (LI) he is at risk of dehydration.
They are trying to slow down the output, thus take away some of the risk. Drinking water can help, and that's all he drinks, but it's not enough. He can only tolerate so much water by mouth too, the IV's were helping to keep him hydrated before.