Um, to the point, yes there is such a thing as too much water. I think 20-30 ml is too much at one time. Split it into AM and PM doses of 10-15 ml. All at once may can dilute his blood's electrolytes and harm him. But, we're dealing with dehydration, which must be fixed. With humans, drinking two gallons of water in 1 1/2 hours can kill. Some stores carry electrolyte-loaded capsules to add to water for people whose water is too pure or is RO water. Calcium, potassium, and sodium make up the most of what is needed in water. There is hyper-tonic(more concentrated than normal good water), which then draws water out of the body which can be harmful by inflating the blood volume. There is hypo-tonic which is less concentrated than the blood, and is safe to drink/feed. I would want you to ask your vet about how much plain water to feed per kilogram of body weight, and how much electrolyte to add to his water. He/she will already know how much electrolytes are in your water supply. I think adding electrolyte would be good for his large daily consumption of water.