Well, there's more ways to get water than from electric pumps.
You are correct that dehydration will kill many in the first week. Next would be the sick and elderly who rely a daily cocktail to stay alive. About that time starvation will be hitting many which will lleave the cities as giant cesspools filled with the dead and rotting bodies. To be followed shortly by massive epidemics and plagues as the mice, flies, and other vermin begin spreading diseases.
"And if the Jamaican pirates don't get you, it'll be the cold embrace of the sea, and that's no lovers kiss."