In the New York Times today, the claim that Caffeine causes dehydration was debunked. They reviewed over ten studies comparing caffeine with water or placebo and seldom found a statistical difference in urine volume.
“In the 10 studies reviewed, consumption of a caffeinated beverage resulted in 0 to 84 percent retention of the initial volume ingested, whereas consumption of water resulted in 0 to 81 percent retention.”
So my 8 diet cokes per day are like 8 glasses of water per day. I can live with that.