The following summary of the international laws concerning naval blockades is by Marc James Small (firstname.lastname@example.org) and appears here by permission.
The Law of Blockade is governed by two treaties, the first being the Treaty of Paris from 1856 which established the basic principles and to which all major nations adhered save for the US, who did agree to be bound by its terms. (This is the treaty which outlawed privateers, incidentally.) Following the two Hague conferences, it was decided that a further compact was necessary, so a small conference was held in the UK, which resulted in the Declaration of London in 1910; in this case, while all other powers signed it, the British did not, but, as had the Americans in 1856, they agreed to abide by its terms.
The basic scheme is that a blockade, to be legal, MUST be effective: the term is a bit hazy, but, essentially, means that the bulk of enemy and neutral merchant shipping must be subject to being stopped. Once the blockade is LEGAL, then a belligerent ship may STOP a neutral or enemy merchant vessel and inspect it. If the cargo is found to be actual or conditional contraband, it can be seized without payment, though this can only be done once the ship is taken to a friendly port (which may be neutral) and a judicial proceeding is held to confirm that the cargo was indeed "contraband". (If the cargo is not contraband but is, for instance, food, it must be permitted to pass unhindered.)
If the blockade is not a legal and effective one, then a belligerent ship may still stop the merchant vessels of an enemy power, though not those under a neutral flag. The enemy warship may still seize the merchant ship and send it to a friendly port, but they must then PAY the owners for the cargo.
You cannot just SINK an enemy ship, under either of these scenarios, unless it attempts armed resistance or flight and, even then, you must make all efforts to use less than deadly force to stop the merchant vessel, such as firing a shot across the bows. You may sink the merchant ship if, and only if, it is in real and imminent danger of recapture by the other guys.
If you harm the crew or passengers on a merchant vessel, you must provide adequate medical care. If you sink the ship, you must place the crew and passengers "in a place of safety", which is not abandoning them on the high seas in lifeboats. (You may TOW the boats, though, within sight of land and give them a course to the nearest hamlet or village, and then cast them off.)
You will note that both sides violated these rules cheerfully and frequently in both wars, though the violations by one side do not relieve the other of adhering to these provisions. At Nuremburg, the British wished to push the German violations in the specification of charges, but the US ensured that the overall scheme was not reviewed by that tribunal, as we were concerned lest Lockwood, Nimitz, Dick O'Kane and so forth also found themselves in the dock. So, this remains the law today.
In the case of LUSITANIA, U-20 committed a number of violations -- she was not part of a legal blockade, she did not order LUSITANIA to heave to, she did not ensure that the crew and passengers were in a place of safety, she did not render medical aid, she sunk LUSITANIA although she was making no effort to escape or resist nor was she in danger of recapture, and U-20 employed deadly force without first attempting lesser means to stop the British ship. I recognize the obvious, that U-20 simply COULDN'T do these things, as it would have meant her own destruction. But, might does not make right, nor does necessity excuse a succession of war crimes.
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