You should refer to your purchase contract as well as the seller's disclosures provided to you at the time you made the offer.
Generally that is one of the questions that is listed on a seller's disclosure. The seller is supposed to answer this question to the best of their knowledge (Yes, No, or Unknown), however the disclosures I've seen generally have a disclaimer that says that the disclosure is not a guarantee of any sort and that the buyer should independently verify all information before purchasing.
This is my two cents and is not meant to be legal advice whatsoever:
It's possible the seller would not be aware of being in a flood zone. If they inherited the property, paid cash for it, or otherwise did not have a mortgage on the property, then they would not have a lender hounding them to require flood insurance. So they may just have been numb to the fact it was in a flood zone. They probably would have answered No or unknown. In this case they have done all they need to do and the responsibility lies fully with the buyer. If, however, they answered No and they knew fully well that the home was in a flood zone and would require flood insurance then that is an intent to deceive you. If you can prove they intentionally deceived you then that would probably be grounds to back out of the transaction and get your deposit back. If not, then you can ask to back out, but unless the seller otherwise violates terms of the purchase agreement then they can probably elect to keep your deposit or not let you out of the agreement.
You should rely on your Realtor to help you with this, and the lesson learned here as always is LET THE BUYER BEWARE.
I hope this helps!