UPDATE, 9:14pm: Ok, mystery solved! The original Facebook poster misspelled Cuellar's name as 'Dan Cuellar.' Dann Cuellar is a reporter with WPVI in Philadelphia. It turns out, as we hypothesized, that it was taken from the Flagship Hotel. (We got there by noting that it was quite similar to this image geotagged from that same spot, although minus the flooding, of course). Cuellar's Twitter page has various other shots from the same location.
One of the reasons I'm willing to do so is that other photographs from the early high tide in Atlantic City corroborate that this area of the city was badly flooded. With Bing Maps, I was able to view the streets in this area and find plausible vantage points from which the photos could have been taken. (Google Maps did not have coverage of the area for some reason.) There is a tall building in just about the precise location that this photograph would have to be taken from. So, again, I'm wouldn't bet my life on it, but I think there is a very good change this photograph is real. If it's not, you should probably hire its creator for your next CGI spectacular.
And finally, Garance Franke-Ruta sent this one over, which didn't take too much investigation.
Thanks to @nathanjurgenson, @edyong209, @discoverymag, @neve_science_wx, @kathyf, @KateRoseMe, @harmonicait, @NigadamaSoup, and @sebprovencher for their help rounding up images.