When Israel takes it upon itself to send in the Marines when Customs is the relevant agency, they will get little sympathy for their approach. This shouldn't have been a military operation.
Anyone with sense realizes that there isn't anything on these relief ships that threatens Israel's security. Wheelchairs and paper and building materials and toys don't threaten the IDF. Israel needs to reject the premise that anyone who disagrees with them (or supports people under effective IDF control) is supporting terrorism and their destruction.
Whether a legal argument can be made that it was appropriate for Israel to commandeer the boats is not relevant, IMHO. The raid was stupid and counter productive to Israel's interests at the very least. As a result, dozens of people are dead and injured and Israel has achieved no improvement in their "security". Instead, they've inspired more people of all stripes to resist the blockades. So Israel will have to commandeer more boats, wasting more resources and risking more casualties, or they have to back down and implement a more sensible policy. There's no effective upside on their current path.
What is it going to take for the people in charge in Israel to stop viewing the world through exclusively military lenses? I dunno. But it needs to happen.
Not that I expect any changes in Israel's outlook anytime soon. :-(
Cheers,
Scott.