America approved these Declarations, so they're binding US law as well "as part of general international law and customary international law." Past US presidents, including Dwight Eisenhower and Jack Kennedy, called blockades acts of war. So has the US Supreme Court.
Occupied Palestine poses no threat to Israel. In the past, Israel admitted it. As a result, imposing a blockade violates the UN Charter and other international and US laws. It's also an illegal act of aggression that under the Nuremberg Charter constitutes the "supreme international crime against peace."
Last September, the UN Human Rights Council (HRC), issued damning findings, "conclud(ing) that a series of violations of international law, including international humanitarian and human rights law, were committed by the Israeli forces during the interception of the flotilla and during the detention of passengers in Israel prior to deportation."
By imposing an illegal blockade, Israel willfully and maliciously caused a grave humanitarian crisis, affecting nearly 1.7 million Gazans, mostly civilians. Aid is vitally needed. Blocking it is a crime against humanity. Moreover, Israel's international waters interdiction was piracy.
A "vessel on the high seas (posing no threat) is subject to the exclusive jurisdiction of its flag State."
Under the laws of armed conflict, a blockade is also illegal if:
(a) its sole purpose is starving the civilian population or denying it other essentials for life; or
(b) the damage to civilians is, or may be expected to be, excessive in relation to the concrete or direct military advantage anticipated.