International law is very clear regarding belligerent parties when the belligerents are countries, but the situation in Gaza is hazier since we have a non state entity which has de facto and de jure control over Gaza. When Israel left the Gaza strip and turned over control to the PA under International law it was no longer an occupying power. Hamas then overthrew the PA administration in a bloody coup and has had complete control over Gaza ever since. Hamas has adamantly refused to acknowledge the right of Israel to exist and has actively engaged in hostile and deadly acts which would constitute acts of war if it was a state. Collective punishment refers strictly to occupying powers which Israel has not been for several years. During times of war belligerents have the right to establish naval blockades of the enemy. The blockading party has the right to board and inspect any ships trying to enter the blockade area to search for contraband. The blockader may deny entry to any ship carrying contraband which (in every definition) includes materials which may aid in the ability to wage war. There is no obligation to allow entry to a ship which has a mixed cargo of humanitarian aid and contraband. Furthermore, the blockader has the absolute right to sink any ship trying to run the blockade. If the ship is sunk the only obligation is to rescue the survivors, which means an active rescue, not passive such as leaving parties with lifeboats or in lifeboats.
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from the article: Israel prepping to block next Gaza flotilla