The only defense of a person responsible for an actual or threatened unauthorized discharge of oil shall be to plead and prove that the discharge resulted solely from any of the following or any combination of the following:
(1) an act of war or terrorism;
(2) an act of government, either state, federal, or local;
(3) an unforeseeable occurrence exclusively occasioned by the violence of nature without the interference of any human act or omission; or
(4) the wilful misconduct or a negligent act or omission of a third party, other than an employee or agent of the person responsible or a third party whose conduct occurs in connection with a contractual relationship with the responsible person, unless the responsible person failed to exercise due care and take precautions against foreseeable conduct of the third party.