Each situation can be different, depending on whether the flooded area was "ponded" (water from the immediate area) or "flooded" (water from outside the area).  If the latter, it may contain both chemical contaminants and human pathogens.  If the former, evaluate whether a septic system may have been compromised, whether any animal feces may be present, etc.  See https://extension.sdstate.edu/flooded-gardens for more information.