Argument: In conflict with Hamas, raid in international waters was legal
Clause 67 of San Remo does allow interdiction of neutral ships in a war among states if, as the clause spells out, the ship is “reasonably” suspected of “breaching a blockade.”
Allen Weiner, a former State Department lawyer and legal counsel at the U.S. Embassy at The Hague, said the raid was lawful to the Washington Post: "Israel claims to be in a state of armed conflict with a non-state group, with Hamas in Gaza. Under the laws of war, a blockage is legal. That includes operating on the high seas. You don't have to wait until you are on territorial waters."
Leslie Gelb. "Israel was right." The Daily Beast. May 31, 2010: "Israel had every right under international law to stop and board ships bound for the Gaza war zone late Sunday. Only knee-jerk left-wingers and the usual legion of poseurs around the world would dispute this. [...] Israel has every right to protect itself under international law, including by blockades in international waters."
"Israel obeyed international law: Legally, the Gaza flotilla conflict is an open-and-shut case." The Daily News.com. June 2nd, 2010: Although the wisdom of Israel's actions in stopping the Gaza flotilla is open to question, the legality of its actions is not. What Israel did was entirely consistent with both international and domestic law. In order to understand why, the complex events at sea must be deconstructed.
First, there is the Israeli blockade of Gaza. Recall that when Israel ended its occupation of Gaza, it did not impose a blockade. Indeed, it left behind agricultural facilities in the hope that the newly liberated Gaza Strip would become a peaceful and productive area.
Instead, Hamas seized control over Gaza and engaged in acts of warfare against Israel. These acts of warfare featured anti-personnel rockets, nearly 10,000 of them, directed at Israeli civilians. This was not only an act of warfare, it was a war crime. Israel responded to the rockets by declaring a blockade, the purpose of which was to assure that no rockets or other material that could be used for making war against Israeli civilians were permitted into Gaza.
Israel allowed humanitarian aid through its checkpoints. Egypt as well participated in the blockade. There was never a humanitarian crisis in Gaza, merely a shortage of certain goods that would end if the rocket attacks ended.
The legality of blockades as a response to acts of war is not subject to serious doubt. When the United States blockaded Cuba during the missile crisis, the State Department issued an opinion declaring the blockade to be lawful. This despite the fact that Cuba had not engaged in any act of belligerence against the United States. Other nations have similarly enforced naval blockades to assure their own security.
The second issue is whether it is lawful to enforce a legal blockade in international waters. Again, law and practice are clear. If there is no doubt that the offending ships have made a firm determination to break the blockade, then the blockade may be enforced before the offending ships cross the line into domestic waters. Again the United States and other Western countries have frequently boarded ships at high sea in order to assure their security."