1) Your homeowners policy doesn't cover floods. Flood damage is not a covered peril on standard homeowners policies and most commercial policies, although many people assume that it is. That can be a costly assumption.
2) You can get an estimate of your property's flood risk online with a "one-step flood risk profile."
3) You may have to have flood coverage. Mortgage lenders often require flood coverage if a home is located in a flood-prone area (also known as a "special flood hazard area.")
4) Most people buy flood coverage through the government. Flood insurance is widely available through the National Flood Insurance Program, which is run by the Federal Emergency Management Agency, or FEMA. There are limits, however, on how much damage they'll cover. Many local agents sell NFIP policies.
5) Rates may be going up. In July 2012, Congress passed the Biggert-Waters Flood Insurance Reform Act, which will change the way the National Flood Insurance Program is run. Among those changes: premiums will increase for some policyholders. That's being done to make the program more financially stable.