Also why just flood risk? Is it unethical for me to sell my Condo which is in “up and coming area” which never upped and never came and has a very high crime rate (with/without disclaimer)? My friend lives in another area where schools are as bad as it gets, she is looking to move now, unethical to sell that too (with/without disclaimer)?

The difference is that schools, crime, etc., are all what they are right now. It's there, it's verifiable. Anybody buying in has access to the full information. They can walk around the neighborhood and see for themselves.

The flooding and inevitable destruction of the city is decades away. It's still abstract. Some people might even think it is preventable.

I don't think it's unethical to sell. People have their own motivations. Maybe a buyer just wants it for 5 years, who knows. Probably the risk will get baked into market price. What does need to happen though is the federal government needs to step up, because they're the only ones who can, and guarantee they will buy it for a certain percentage of appraised market value. I would imagine that percentage will decline over time until they declare the city a total loss, after which your property is declared worthless. If they do this now, they can make it possible for people to leave with some semblance of dignity and mitigate hardships.

Is it unethical to lie in order to sell something? Yes, yes it is.

This sort of puffery is relatively minor and is thus not tremendously unethical, but it is unethical.