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.
Voluntary flood insurance policies are not enforced until 30 days after purchase.
Flood insurance is required by law for some high risk homes but only a minority of homes fall under this category. Congress passed some laws requiring that all federally-insured or regulated lenders, for instance mortgages backed by Fannie Mae or Freddie Mac require flood insurance to protect their investment in the property.
Overflows from sewer systems or sump pumps are not considered a flood unless a flood has caused the backup and overflow.
Flood damage is not typically covered by a homeowners insurance policy.