You rock. It's interesting that blockade-running is a lasting technique of protest; Joseph Conrad did a spot on behalf of the Spanish Carlists, Erskine Childers for the IRA (although he thought he was doing it for the other side), and then, there was the saga off Bilbao in the Spanish Civil War. It's always worth doing to go and probe what the actual limits to freedom are; naturally, they are set by the other side's available will.
This tends to vary between the sea and the land. Winston Churchill wrote, about the German decision for unrestricted submarine warfare in 1916, that no-one would have objected had a lot of neutrals been driving trainloads of war supplies up to the front and the Germans had turned their artillery on them. Sinking neutral ships, however, was somehow a lot more offensive. Similarly, it seems that the Israeli military finds it easier to use force at some sweating checkpoint than on the high seas; foreign nationality hasn't always been protective.
A couple of possible reasons come up; one is that (as Churchill suggested) it simply feels and looks awful, to the people who would have to carry it out. Especially as one of the great Israeli historical grievances regards the Royal Navy intercepting immigrant ships at sea. (Why else did the Foreign Ministry mouthpiece say they didn't want a "well-publicised provocation in the middle of the sea"?) A second, related, is that the sea is for everyone, like the radio spectrum. Crucially, with 20 miles or so of shore to aim for, and territorial waters used by quite a lot of other small craft, it would have been hard to spot two more wooden boats at night, so an arrest would have to happen further out at sea, in which case it would have been on the high seas, in international waters. Major sea powers tend not to like this, especially when they already have a grudge on the matter. Further, it would have been a precedent for other navies in the area.
There's another point, of course; the idea of a naval blockade has traditionally been financial and legal. If there was an "effective" - we'd now say credible - blockade, a ship that breached it invalidated its insurance, and that of its cargo. Similarly, the blockade invoked the force majeure clause in any contract that required goods to be shipped through it. A little force went a long way. This was the situation off northern Spain at the end of 1936; the Fascists had a few ships in the area, and wanted to prevent shipping reaching the Republican-held ports there. They had the further problem that no-one recognised them as having belligerent rights - i.e. unless they could stop ships, the blockade would not be legitimate.
The Royal Navy was in the area, but wasn't keen on picking a fight with the fascists, largely because the British ambassador to Spain was getting his information from them. Dozens of ships with cargos for Bilbao were held in Bordeaux by the blockade; one of them was about to call the bluff. The Seven Seas Spray sailed in defiance of the blockade, and all advice, and arrived in Bilbao. The next shp to go was intercepted by the fascist-controlled cruiser Almirante Cervera. She instantly radioed for the Navy's assistance; she was a British-flagged ship, and so the Navy had an obligation to defend her. The British government had tried to balance believing in the fascist claims by sending more ships; the Hood, Resolution, and a gaggle of destroyers arrived. What was more, the Hood's 8 15-inch guns were trained on the Almirante Cervera.
They didn't discuss it further. More importantly, the blockade was over; there were no mines, and the threats were empty, and the Royal Navy was now unwillingly committed to protect any shipping in that sea. The roadstead in the Gironde emptied. All it had taken was the willingness to defy; probably, with more effort, the fascists could have sunk a ship, but they were not in a position to stop them all.
Outside Dubrovnik in 1991, there was a more postmodern take on the tradition; a bizarre gaggle of journalists, intellectuals, pacifists, and crisis tourists decided to run the Serbian encirclement of the city in a chartered Ro-Ro ferry. Despite being stopped by an (ex-)Yugoslav Navy vessel, they were eventually allowed to pass after a highly surreal parley; the downside was that there wasn't actually very much in the way of aid aboard the ship or aboard any other ship. There is a telling account of the journey here; note that one prediction from it was very much true, as Stipe Mesic did indeed get to be president of Croatia.
I have to say that the makeup of the Gaza convoy isn't that promising - Yvonne Ridley? Was George Galloway unable to open his sunbed? But the spirit is right, and they strike an important point - Israel does all its trade through its ports, over the sea. They can afford to start a row about the freedom of the seas as much as Singapore or Britain can.