According to Israeli law, under the adoption of British Mandate Laws related to naval interdictions/blockades, Israel cannot simply "take" the items and transfer them to Gaza. This is not legal because taking possession of a cargo is considered piracy without proper procedure. Israel is required under IT'S OWN LAW as adopted by the High Court, to conduct a "Prize Court" judicial procedure in order to have the rights to take/transfer possession of the cargo. As for the international law matter, if the siege of Gaza is against Geneva 4 etc., then humanitarian aid is legal in the face of the military blockade. Israel can, of course, interdict these ships, but how they do it and what they do is tightly prescribed. From what they have said publicly, it appears they aren't aware of their own laws. The court of world opinion will judge their piracy.
4 UN peacekeepers injured in Sinai and taken to hospital in south Israel (Haaretz)
from the article: Israel Navy commandos: Gaza flotilla activists tried to lynch us