There are two markets and they differ
Flood can be bought through the federal program or from private carriers. The federal policy has standardized limits and terms; private flood policies frequently offer higher limits, sometimes broader cover and sometimes a shorter waiting period. For a property above the federal building limit, the private market is worth looking at rather than assuming one option exists.
The waiting period is the part people lose on
Flood cover commonly does not take effect for a set period after purchase, specifically so it cannot be bought against a forecast. On a river that rises with a storm upstream, that makes this a decision for a dry month. There are limited exceptions, and none of them help the week it rains.
Building and contents are separate purchases
A flood policy covers the structure and covers your belongings as a separate election with its own limit. Buying building cover alone is common and leaves everything inside uninsured. Below-grade areas are also treated restrictively, which matters where a property has any sunken space.