Before I get into any discussion, I want to go over one thing. According to
"San Remo Manual on International Law Applicable to Armed Conflicts at Sea", assuming the Israeli blockade of Gaza is legal, then the interception and boarding of these ships is legal. The Gaza blockade is well known and all the ships involved were given fair warning that they'd be intercepted for trying to breach the blockade. These terms allow Israel to intercept ships in international waters to preempt them from breaching the blockade (if it is a legal blockade).
What I'm curious about is whether this blockade is legal or not (keep in mind that legality is very loosely used considering how weak "international law" actually is).
These are two interesting articles I found in a quick search of the matter. They both discuss the same issues. The first argues that the blockade is illegal, and second argues it is legal.
http://opiniojuris.org/2010/06/02/why-is-israels-blockade-of-gaza-legal/
http://online.wsj.com/article/SB10001424052748704025304575284210429984110.html
There are two main issues regarding the question of legality. First, accepted international law defines war as an armed conflicted between two sovereign states, and that state actors can legally blockade each other in war. The second is that Gaza is not a sovereign state, but there is obviously some sort of armed conflicted between Israel and Gaza. There is little historical precedent for this sort of situation, making for no clear answer.
Both articles discuss the Union blockade of the Confederate States during the American Civil War. The CSA were not a sovereign state, so the Union blockade was initially called illegal by ships that were intercepted. The US Supreme Court decided an armed conflict existed, so it was ok to blockade. The second article asserts that this implies a state's right to use blockades against external AND internal threats. The first article, though, suggests that the declaration of a blockade and armed conflicted gave recognition to the CSA as a legitimate sovereign state, meaning there is a legally defined war occurring, hence the blockade was legal.
The point the first article makes is that Israel denies it occupies Gaza while refusing to recognize it as a sovereign state. The author suggests that if Israel switched either position, the blockade could be legal, but currently is not because it does not meet any standard for an international armed conflict. The second article asserts the Supreme Court decision for the Union blockade is precedent enough for the Gaza blockade to be legal. So we still have no clear answer here.
In January of 2009, the UN Security Council passed
Resolution 1860 nearly unanimously (the US abstained), which called for a ceasefire to the Gaza War and the end of the blockade. Because Israel is a party to the United Nations Charter, it is bound to follow any order from the Security Council. By this reasoning, the blockade is now illegal if it was not before. (
NOTE: Entering personal opinion, I have not been extensively educated in international law) The ceasefire called by the Resolution was not followed by Israel OR Hamas. If it had been followed by Hamas, Israel would be directly bound to follow it as well, but Hamas did not, leaving Israel with no expectation of following it as well.
Personally, Israel's use of the blockade is a poor choice, legal or not. It could legally wait for ships to arrive in port and then search what they carry. It could also allow far more humanitarian supplies into Gaza while serving its purpose of blocking the flow of weapons. THAT BEING SAID, I believe the Israel blockade should be declared legal, for the precedent of declaring otherwise does not seem favorable. Imagine a state facing a serious internal conflict not being able to intercept shipments to its ports because they are not at war with a sovereign state. Imagine if the Union was unable to enforce its blockade (a large reason the CSA lost the war) because it was not at war with a sovereign state. While it might not be the most favorable decision in this situation, it seems to me the best overall. If this does not satisfy the situation, the UN Security Council would still be free take action regarding the humanitarian crisis in Gaza and Israel's arguably disproportional military action.