You don't necessarily need to walk to get to cisterns and rooftop tanks; presumably they are located around the city for people's convenience...
And the food for Waterdeep is grown locally - they have several thousand square miles of farmland around the city, from the Dessarin to the east, to the coast to the south and west, to the environs of Amphail to the north. On top of that, Goldenfields provides a lot of food that's sent by wagon daily to the city (one book, SKT maybe, talks about the lines of wagons that make the trip). There would have to be grain silos for storage somewhere, probably scattered throughout the countryside, but close enough to ship in when grain stored in them is needed. Or maybe in some area just off the maps in a huge silo complex.
Some quick calculations: it's an approximately 75 mi x 75 mi area here (Amphail is "three days ride" from Waterdeep, and a day's ride is usually around 24 miles) , or 5625 square miles. There are 640 acres in a square mile, so 3,600,000 acres. In medieval times, an acre of land in England produced 7 to 15 bushels of wheat per acre (
Agriculture in the Middle Ages - Wikipedia ), so let's go with 10 as an approximate average. That's a potential of 36 million bushels of wheat per year. Each bushel will be 60 lbs of wheat (so 2,160,000,000 lbs of wheat). We'll turn those pounds of wheat into pounds of bread (although the addition of water and other ingredients means that it's not an exact equivalent) - each pound of bread contains approximate 1000 calories, and an adult needs around 2000 calories, or approximately 2 loaves of bread per day, or 730 per year. 2,160,000,000/730 = 2,958,904 adults that can be fed if the entire area is used for farmland. That's enough to feed the city and the population around to actually grow the crops. Presumably due to magic, yields, especially from Goldenfields, will likely be much higher than this. Conversely, obviously some of the land will not be arable, or would be used for other purposes, probably even a substantial percentage. But it definitely looks like the city can easily feed itself just from its environs. As a side note, aren't Imperial units fun lol.