Maybe the mayor misspoke and meant that 80% of the city is below sea level, not underwater. That makes it a reminder of the difficulty of the situation, not breaking news, and can explain the nonchalant way in which he said it.
If you had been watching the video feeds from the local TV station today then you would find it easy to believe that 80% of the city is flooded to some level. Trust me there is a lot of water around there.