International law experts differ over the legality of the Israel action, with some saying that the raid is a violation of the Law of the Sea, while others maintain that Israel may legally board foreign vessels in international waters as part of a naval blockade. Both sides agree that Israel is required by law to respond with the proportional use of force in the face of violent resistance.
Legal opinions supporting the action
Mark Regev, spokesman for the Prime Minister of Israel, referring to the San Remo Manual on International Law Applicable to Armed Conflicts at Sea, said:
“ The San Remo memorandum states, specifically 67A, that if you have a boat that is charging a blockaded area you are allowed to intercept even prior to it reaching the blockaded area if you've warned them in advance, and that we did a number of times, and they had a stated goal which they openly expressed, of breaking the blockade. That blockade is in place to protect our people. ”
The non-binding San Remo memorandum (paragraph 60, chapter e) states that refusing an order to stop or actively resisting visit, search, or capture may render merchant vessels military objectives. Paragraph 47, chapter c, states that vessels engaged in humanitarian missions and carrying supplies indispensable to the survival of civilian population are exempt from attack, on condition they were operating based on "agreement between the belligerent parties", but the government of Israel has indicated that it had not agreed to the vessels breaking the blockade.
"The Israeli blockade itself against Gaza itself is not illegal, and it's okay for Israeli ships to operate in international waters to enforce it," said Allen Weiner, former U.S. State Department attorney and legal counselor at the American Embassy in The Hague, and now a Stanford Law School professor.
According to Abbas Al Lawati, a Dubai-based Gulf News journalist on board the flotilla, Israel is likely to cite the Gaza–Jericho Agreement (Annex I, Article XI) which vests Israel with the responsibility for security along the coastline and the Sea of Gaza. The agreement stipulates that Israel may take any measures necessary against vessels suspected of being used for terrorist activities or for smuggling arms, ammunition, drugs, goods, or for any other illegal activity.
Tel Aviv University law professor Yoram Dinstein has written that "there are several instances of contemporary (post-UN Charter of the Law of the Seas) practices of blockades, e.g., in the Vietnam and in the Gulf War."
Legal opinions opposing the action
Richard Falk, U.N. Special Rapporteur on the situation of human rights in the occupied Palestinian territory said that the “ships were situated in the high seas where freedom of navigation exists, according to the law of the seas” and called for those responsible to "be held criminally accountable for their wrongful acts".
In a legal analysis published by the Frankfurter Allgemeine Zeitung, a staff expert on international law explained that countries are not allowed to extend their sovereignty on areas outside of their coastal waters. In a zone extending 24 nautical miles from the coast, countries have the right to inspect ships in order to enforce immigration and public health laws and regulations. In international waters, if there is reasonable suspicion of piracy or human trafficking, a country has the right to access foreign ships. If the suspicion remains, it can search the ship. Israeli soldiers have the right to defend themselves. If Israel has used force against the ships without legal justification, the crew members had the right to defend themselves.
Robin Churchill, international law professor at the University of Dundee in Scotland, said there was no legal basis for boarding the ships as they were in international waters.  Ove Bring, Swedish international law professor, said that Israel had no right to take military action. That was supported by Mark Klamberg at Stockholm University. Hugo Tiberg, maritime law professor and Geir Ulfstein, professor at maritime law at University of Oslo while Jan Egeland, director of the Norwegian Institute of International Affairs said that only North Korea behaved in international waters in the same manner as Israel. Canadian scholar Michael Byers notes that the event would only be legal if the Israeli boarding were necessary and proportionate for the country's self defence. Byers believes that "the action does not appear to have been necessary in that the threat was not imminent." Jason Alderwick, a maritime analyst at the International Institute for Strategic Studies of London, was quoted as saying that the Israeli raid did not appear to have been conducted lawfully under the convention. Anthony D'Amato, international law professor at Northwestern University School of Law, argued that the San Remo Manual on International Law Applicable to Armed Conflicts at Sea applies to a situation in which the laws of war between states are in force. He said the laws of war do not apply in the conflict between Israel and Hamas, which isn't even a state. He said the law of the Geneva Conventions would apply. Said Mahmoudi, expert on international law, said that boarding a ship on international waters, kill and capture civilians is not in line with the law.
A group of Israeli lawyers, including Avigdor Feldman, petitioned the Israeli High Court charging that Israel had violated the United Nations Convention on the Law of the Sea by capturing the boats in international waters.  In response to the petition, Israel's legal team wrote that "the petition suffers from a fundamental distortion in the description of the events as they actually transpired in reality, to such a degree that it is unclear what relationship there is between it and the possibility of doing justice, as the petitioners claim to seek in their action."
Turkey's foreign minister called the raid "a grave breach of international law and constituted banditry and piracy—it was “murder” conducted by a State, without justification". Prominent Turkish jurists have characterized Israel's actions as a violation of international law and a "war crime." Dr. Turgut Tarhanlı of the University of Istanbul cited the concept of innocent passage, under which vessels are granted safe passage through territorial waters in a manner which is not "prejudicial to the peace, good order or the security" of the state.
Dr. Turgut Tarhanlı, from the law department of İstanbul Bilgi University, said
the Convention on the Law of the Sea stipulates that a coastal state may consider intervention if a ship is engaged in arms and drug smuggling, the slave trade or terrorist activities. However, the case with the aid boats is totally different. They set sail in accordance with the Customs Act and are known to be carrying humanitarian aid, not weapons or ammunition. According to the Convention on the Law of the Sea, Israel was not entitled to launch a military operation against the boats and activists