Flood damage is not typically covered by a homeowners insurance policy.
There are some homeowner endorsements that will cover a back-up or overflow of sewers or drains or a sump-pump. These type losses sometimes do occur during flooding events. These endorsements will often have dollar limits substantially less than the flood policy
If a house is destroyed due to the collapse of the land it is built on, and the collapse is due to waves or currents of water, that would be considered a flood and would be covered.
Overflows from sewer systems or sump pumps are not considered a flood unless a flood has caused the backup and overflow.
Voluntary flood insurance policies are not enforced until 30 days after purchase.