What can we do as owners?

You could potentially sue the realtor if you can show that he or she KNEW the land flooded but lied to you about it. They are only potentially liable if they had knowledge of the flooding, since any liabilty is based on "fraud"--basically lying--and you can't lie where you don't know the truth yourself. So if the realtor was told by the homeowner or city that there was no flooding, there was no lie--the realtor told you what they knew to the best of their knowledge.
Similarly, if you can show that the seller lied about the degree of flooding--that the seller know how often and how much it flooded--and that the flooding is serious enough to be considered "material" (an important or significant consideration in somene deciding to buy), you could sue the seller. You'd have to show that either they did know or that any reasonable seller in the seller's position (e.g. anyone who had lived there as long as they did, assuming they lived on the property) must logically have known--but that despite knowing, they failed to disclose it to you, thus effectively lying about it.