The question at hand: In-universe, why is "Waterdhavian" the adjective form of Waterdeep?
This question seems to be very close to a designer intent question (which are off topic). It does not fit the definition perfectly because it is not asking about the origin of why a rule is a certain way. Instead it is asking why a certain design decision (in this case a name) is a certain way. I'm not sure if the fact that they are asking for an in-universe explanation helps it to be further away from a designer intent-type question or not, but it actually feels like it does for some reason.
Regardless, and, I think, significantly, it seems to be displaying at least one of the same issues that designer reasons displayed. Many of the answers don't seem to be citing any sources and seem to be relying on speculation instead. (Disclaimer: I have since added an, hopefully properly-evidenced, answer to the question.)
So, by my reckoning, at the very least we have an issue with answers. But when we have several answers all displaying the same problem I begin to smell an issue with the question itself.
Is this question on-topic?
If it is on-topic, what, if anything, should we do about the answers currently under the question?
Note: I have since added an answer to the question since I found the answer while looking around for it. I still think the question might be off-topic, but I didn't think it would be improper to give an answer while the community discusses it.