(a) The President, through any agency of the Department of Defense designated by him, may arm, have armed, or allow to be armed, any watercraft or aircraft that is capable of being used as a means of transportation on, over, or under water, and is documented, registered, or licensed under the laws of the United States. (b) This section applies during a war and at any other time when the President determines that the security of the United States is threatened by the application, or the imminent danger