I agree with point 3, as, like you suggested, private firms are generally more efficient - partly due to the profit motive. Therefore, if the flood defences are provided by a private firm, it is likely that they will be of better quality for the aforementioned reasons.
A point to argue for private funding could be that it will mean that the government will not have to spend on flood defences, meaning that there is no opportunity cost for the government and thus it can spend on other things, such as defence, or alternatively it can not spend the money and therefore reduce the budget deficit.