dry fasts aren't always what they appear. if you have significant glycogen stores in your body as you begin your fast you wont be dehydrated for the first day or two as water is freed. what usually happens is someone who starts glycogen endowed discovers that they aren't thirsty when they start fasting and tout it as dry fasting.

Religious Jews have a couple of 25 hour complete fasts per year.

Look up how Muslims fast during Ramadan.