Is It Always Illegal to Board a Ship in “International Waters”?

According to Glen Greenwald:

It hardly seemed possible for Israel -- after its brutal devastation of Gaza and its ongoing blockade -- to engage in more heinous and repugnant crimes.  But by attacking a flotilla in international waters carrying humanitarian aid, and slaughtering at least 10 people, Israel has managed to do exactly that.  If Israel's goal were to provoke as much disgust and contempt for it as possible, it's hard to imagine how it could be doing a better job.

The claim that there's something illegal about boarding ships in “international waters” appears to be the centerpiece of the current outrage. On the contrary, in another context:

Drug smugglers who ship tons of cocaine in on handmade subs are about to get the U.S. Coast Guard treatment. Because of a loophole in U.S. maritime law, the orange navy can't stop unflagged ships in international waters, meaning that these little subs and semisubmersibles can float legally right up to our waters. But new legislation OK'd this week in the House and set for consideration in the Senate will let federal authorities stop unflagged vessels in international waters. It's an antiterrorism and antidrug issue that came to Washington's attention when handmade drug subs loaded with 12 tons of coke started showing up.

If anything, the drug smugglers have a better case.