They may have been real. In addition to breeding with humans until there is no example of human DNA in existance that is in its pure original form, the Demigods also crossbred animals. With all the mud that we can see that was deposited in places like the grand canion, it is unlikely that they would have survived the danger of the debris filled turbulent waters of the flood. We should also find them in the layers of mud that we can tell were deposited as the flood waters receded due to the lack of evidence of the forces of erosion on the flat layers of mud. In the time that it is said that took, the surface of the earth would have washed into the ocean thousands of times over at the current rate of erosion. I can not prove that no mermaids existed or that there are no survivors; however it is known that the premise for the creature comes from the idea that demons bred with creation. If anyone were to find a mermaid alive today, they would most likely be killed by a very evil creature that should not be toyed with.