Yes, the stunt I pulled in college was dangerous, but adding a gallon or even two to a healthy adult's water consumption throughout the course of a day is not. Especially considering the sodium content of modern diets. And those can be supplemented by snacking on crisps, pickles, or salted nuts, if needed.

Water drinking contests, along with overhydration during/after intense physical exertion (e.g. marathons, partying on MDMA), account for nearly all known cases of death from water intoxication.