For example, this description in Reuters:
Andre Abu Khalil, a Lebanese cameraman for Al Jazeera TV, gave an account that backed some of what both sides have said.The International Convention Against the Taking of Hostages starts off this way:
In his telling, activists initially wounded and captured four Israelis from a first wave that boarded the ship. A second wave of troops tried to storm the ship after the four were taken below decks.
One activist used a loudhailer to tell the Israelis the four captive soldiers were well and would be released if they provided medical help for the wounded activists. With an Israeli Arab lawmaker acting as mediator, the Israelis agreed. Wounded were brought to the deck and were airlifted off the ship.
Any person who seizes or detains and threatens to kill, to injure or to continue to detain another person (hereinafter referred to as the "hostage") in order to compel a third party, namely, a State, an international intergovernmental organization, a natural or juridical person, or a group of persons, to do or abstain from doing any act as an explicit or implicit condition for the release of the hostage commits the offence of taking of hostages ("hostage-taking") within the meaning of this Convention.
Any person who:
attempts to commit an act of hostage-taking, or
participates as an accomplice of anyone who commits or attempts to commit an act of hostage-taking likewise commits an offence for the purposes of this Convention.
In other words, the "humanitarian activists" performed a textbook definition of hostage taking. Which means that these "humanitarians" violated humanitarian international law by taking the IDF soldiers hostage.
It doesn't matter if it was only for a relatively short time. It doesn't matter that the hostages were soldiers or that the hostage takers were nominally civilians (although I am fairly certain that by acting violently against the soldiers legally enforcing a blockade, and by taking them hostage, they forfeited their status as civilians and became official combatants, waiving their rights as civilians under international law.) All that is irrelevant to the definition here: this was a case of hostage taking.
This treaty was acceded to (accepted as law) by Turkey in 1989.
Not only that, but hostage taking is a form of terrorism.
This Convention is considered by the UN to be one of the "legal instruments and additional amendments dealing with terrorism." Moreover, it is referenced in the International Convention for the Suppression of the Financing of Terrorism.
This means that whoever was involved in the decision to take Israeli commandoes hostage was, effectively, engaged in terrorism, by the UN's definition and under international law.
IHH seems to have been clearly complicit, but the other groups such as Free Gaza may also be implicated in this terrorist act.
Israel would be within its rights to demand extradition of the hostage-takers, those who aided them, and those who know the identity of the hostage takers, to stand trial for these terrorism charges.