Drinking or not drinking water is not mentioned in the details about the Day of Atonement given in chapter 16 of the book of Leviticus. Benefits of drinking water One benefit of drinking water is being able to fast longer without feeling totally wiped out. The purpose of fasting is to draw closer to God to be strengthened spiritually. Sometimes because of our humanity this takes time. Another benefit is that elderly people and those with serious health problems can more easily fast.
Christians are more easily able to continue working at their job if water is taken.
Whether to drink or not during a fast is a personal decision. Christians are free to drink or not to drink. But we are not free to judge or condemn others and the choices they make. When fasting is done secretly, others will not even know we are fasting or whether or not we are drinking water. It is a private matter between the person and God.