IMO, a property can not have a negative market value if the market value is the assignment. A property can have no value, as in, nobody is willing to pay for it.
A negative value would mean the owners of the property PAY to have someone take it off their hands ( creating a negative value ), which happens in a few cases, but I doubt it applies here.
A LOSS to the owner-seller is not the same thing as a negative value of the property !!! Just as a profit to the owner might not be the market value of the property. MV depends on what a buyer is willing to pay - whether the owner took a loss on the property or not.
What are the other homes on that street or affected by the same flood drainage in the community selling for? If they sell for any $ even if very low, that is the value.