Demetrios1453
Legend
First, most of Waterdeep is on a plateau (everything north of the harbor district) that's somewhat above sea level.Ok, been noodling around with dumpster diving Forgotten Realms and specifically Waterdeep lore. And, I was planning something for use in Waterdeep. I wanted to introduce the idea of Battagliola (https://www.thearma.org/essays/BridgeWars.htm#.ZF7dt85ByUk) or stick fighting/mock combat on bridges in Venice in the late Middle Ages. But, there's a problem. There are no rivers in Waterdeep. None. In a city of several hundred thousand people, there are no actual freshwater sources.
Now, again, noodling around the web, the answer that was given is that Waterdeep has lots of wells. This is a huge problem though for a city that's (mostly) on sea level, any water in a well would likely be salt water. You don't get wells in ports. ((Note, there is an equal problem with the notion that Waterdeep has sewers - but, that's a problem for another day)). So, where do all these people get their water from?
Seems a bit of an oversight really. After all, Baldur's Gate has a honking big river, and so does Neverwinter. But, Waterdeep is nowhere near an actual fresh water source.
Secondly, Waterdeep basically is at the same latitude and has the same climate as Seattle - perhaps even more stormy. So the plentiful rainwater can be stored in tanks or cisterns in significant amounts.
And the final reason is Undermountain. Millennia of dwarven and wizardly engineering projects means that several freshwater rivers flow below the city, some below sea level. Skullport is several levels down, but it's on a freshwater river called the Sargauth. Smugglers go from Skullport upward through magical locks on the river to reach the sea. Now, digging wells into Undermountain might not seem to be the best idea, but sometimes unpleasant but necessary things need to be done.