Depends on where you are. There are public pumps (that anyone can visit to fill water containers), even in Castle Ward. They're the only source of water far from Mount Waterdeep; parts of the city close to the mountain get gravity-fed piped water. (Something my players first…
— Ed Greenwood (@TheEdVerse) October 6, 2018
…asked about back in 1978. That's the same year of Realmsplay in the "home" Realms campaign when a villain tried to poison most of the city by tainting the water supply, got foiled by PCs, and Piergeiron had safeguards (cisterns with testing outflows and shutoffs) installed.
— Ed Greenwood (@TheEdVerse) October 6, 2018