Disclosure requirements vary among states, from the detailed to the bare-bones. See "State-by-State Seller and Broker Disclosure Requirements" for examples. The key question is how much does your state law require the seller to disclose to buyers? If the standard form doesn’t mention past flooding, the seller doesn’t have to, either (but shouldn’t lie if asked). You might want to read your state’s law, or at least the form, to look for holes.

The majority of states require sellers to either fill out a disclosure form or disclose material facts about the property.  To find your state’s law, talk to your real estate agent or state regulatory agency. You can find your state real estate agency at the Association of Real Estate License Law Officials website. Or you can search online for “real estate disclosure,” “disclosure form,” or “disclosure statement” and the name of your state.  Disclosure requirements for specific states, such as Illinois, are also included in the Preparing and Showing Your Home section of the Nolo site.

But even in nondisclosure states, buyers can negotiate to make seller disclosures a part of their purchase—or may get them without asking. Law or no law, your state Realtor’s association has probably created a standard disclosure form for sellers to use; if you can’t find yours, the National Association of Realtors website has a list of associations by state.

Federal Lead Disclosure Requirements
If you are buying a house built pre-1978, federal law requires the seller to give buyers a form disclosing whether there might be lead-based paint in the home and a pamphlet called “Protect Your Family From Lead in Your Home.” For more on the subject, see the Nolo article Seller Responsibility to Disclose Lead-Based Paint Hazards.