Urban Legend explanation:
If you drink too much coke so fast as you drink water, the carbonated water will leave a trail of compressed CO2 down your food pipe and into your stomach and when it releases, it will bloat it up and you will incredulously internally choke to death!
(whoever said these things make sense?)
One tends to drink water fast when thirsty. (A) It’s hard to achieve that without causing a soda-burn to the back of your throat, and (B) even if you do gulp down copious amounts, the belch (!) is going to be a long, disgustingly enjoyable affair much to the consternation and “eeeuuuusss” with obligatory related facial expressions from the females in your company.