Point (1) the Sanremo Manual is for blackades DURING ARMED CONFLICT AT SEA. There is no armed conflict here, and so the manual as a point in law is moot. Point (2) Even if you DO want to use the Sanremo Manual to justify this action you face this problem: the manual says you must ANNOUNCE any changes to your blockade line. So if you want to claim that Sanremo allows you to board a ship OUTSIDE of that Israeli-declared blockade zone then you have CLEARLY violated the requirement that you can not arbitrarily and pre-emptively change where you enforce that blockade.To argue both is to make the manual internally contradictory, and that is what is known as a "manifestly absurd" result.
Key U.S. House Democrat cites troublesome issues in Iran nuclear deal (Reuters)