Law. Of. Proportionality.
Sinking a ship with 600 civilians on it to keep humanitarian goods from reaching Gaza is NOT proportionate. Under any standard.
By the way. Its good to see such a strong believer in laws. Here is one for you:
Paragraph 102 of the Manual prohibits a blockade if "the damage to the civilian population is, or may be expected to be, excessive in relation to the concrete and direct military advantage anticipated from the blockade".
Considering the documented amount of suffering and the clear intention of the blockade to be collective punishment, its clear that the blockade is illegal, and hence so is the enforcing of the blockade.
Laws from different areas cover the same types of situations.
The only laws that apply to this situation are the international laws, so pulling up some law from South Africa about it being illegal to prohibit chocolate is not going to help you here.
Since the blockade has never been challenged in international court, you are going to have a hard time proving that the damage to civilians is anything significant. They are in a de facto war zone, and Israel supplies food, clothing, and medical supplies to Gaza on a daily basis. The amount of cargo on those vessels is insignificant next to what Israel allows in every year, and barely approaches what they let in in a week.
At least you are attempting to keep the argument about the legality of this blockade based on reality, rather than wild and unsubstantiated claims that it is illegal.