I have no clue as to the detail on flood insurance or claims.
In the claims world that I am in, we routinly obtain property damage releases AFTER the owner of said property has been properly indemnefied.
When you sign a release you are saying that you will not hold that person or business to any future claims. If the insurance company did not estimate the damages properly or if the builder goes over the amount the insurance company paid due to unseen damage or materials cost went up, if the release is signed, you might not be able to collect the additional amount it took to put the property back to the way it was preloss.
Check the laws, but I dont think I would sign a release until the job was done and everybody that worked on it was paid.