You can use this press release by the Israeli Ministry of Foreign Affairs as a brief reference.
International Maritime Law regarding blockades is based largely upon some establish treaties, but also largely on convention and tradition. The first established treaty in International Law that defined what a blockade was, was contained in the Congress of Paris in 1856. The main "point" of this Congress in regards to blockades was that the blockading party must have the appropriate level of enforcement to enforce such a blockade...aka. they could not just say they had a blockade and then go and seize any ship that came into it at random. No, there had to be an enforcement capability that restricted ALL ships going in and out of the blockaded area.
International Law has since developed on the issue further...with some questions over what ships are allowed into the blockade (eg. are neutral ships allowed through etc.), and convention and laws have established that it is acceptable for the blockading nation to not allow ANY ships to come in or leave the blockaded area.
Now we get to the specifics of the Gaza/Israel case. First, for a blockade to be legal internationally then the country doing the blockading must be in a state of war with the blockaded country. Israel has legally been at war with Hamas, since the latter organization took control of the Gaza strip.
Next, the blockade must be known by all of the international community...this has been the case since the implementation of blockade in 2007. The whole world knows and recognizes the blockade. -Thats the key...they recognize it, I'm not saying they think it is legitimate.
Next big question...are maritime blockades acceptable in "international waters" or the "high seas" as defined by the U.N.'s Convention on the Law of the Sea. Simple answer is yes. The London Declaration of 1910 contains the limits of blockade in the "waters" surrounding the blockaded party. The full text can be found here...
The "humanitarian flotilla" failed in two distinct ways, and thus broke international law. First, they left the port of a neutral territory (turkey - interestingly the port they departed from was in Turkish OCCUPIED Cyprus...which btw the U.N. has denounced) - see chapter 1:section15 - with the complete knowledge of the blockade...there purpose was to run it, as written memos and oral correspondence has collaborated. Second, they not only left a neural territory knowing that the blockade existed...but as was just stated, they had the sole intention to try to run the blockade...and thus the Israelis were justified in boarding the vessel - Article 1: Section 14-21.
Thus, Israel had the LEGAL right under international law to board the ship in International Waters...the blockade was enforced, the international community was aware of it, and the flotilla left the Turkish port knowing full well that it was going to break the blockade.
My two cents...comment if you like.