Who is legally liable if my realtor did not disclose the fact that my home was is in a high-risk flood area and what can be done about it?

The seller is likely potentially liable, since this is a disclosure that he/she would be required to make, provided that he/she was aware of this fact which is likely, but not guaranteed it is a material disclosure, or one which could reasonably affect your decision to buy and/or price, and the seller may not withhold this information if he/she knew it. You can show that the seller "knew" it by showing that under the facts--e.g. what they would have received from their town or insurer--that any reasonable seller in his/her shoes reasonably must have known.
The realtor would be liable only if the seller had told him/her about the flood zone and despite knowing of this fact, did not tell you it is important enough that if a realtor knew, he/she would have to tell a prospoective buyer. The realtor however does not have any duty to investigate this or would have sources for it other than what the seller provided, and is entitled to rely on what the seller tells him/her that's why, to hold the realtor liable, you'd have to show that the seller actually told him/her the truth but knowing it, the realtor lied or hid the truth.
As to what can be done you may be able to void the sale for fraud--they get home back, you get money back--though this can be difficult to do in home sale contexts, since the transaction is so large and complex, and the seller may not have the money available anymore to return. If they paid off a mortgage, the bank, which is an innocent third party to the transaction, would not have to return the money, for example.
Or you could seek compensation from the seller such as the difference in price between what you did pay and what this home would be worth in a flood zone presumably substantially less.
Given that there is likely a great deal of money at stake, you are advised to consult with an attorney about the situation and your options.