They were boarded far into INTERNATIONAL waters, not Palestinian waters.I just LOLed a little. There is no such thing as "Palestinian waters". Geography, history, and current events FAIL.
I'd advise you read up on on, well, everything, as your ignorance is apparent, as is your editorializing.
Then why worry about the boat getting to Palestine if it physically can’t?
Bodies of water within 24 miles of any country are considered territorially owned and bodies of water an additional 200 miles are more loosely owned and governed. There’s at most 10-20 miles of Israel between Palestine and the Mediterranean Sea.
http://en.wikipedia.org/wiki/International_waters
When the ship got within Israel’s 24 miles (which happens before Palestine’s overlap it), Israel could have done what it did without concern.
However, dennisw has the best information on this in this post:
http://www.freerepublic.com/focus/news/2527527/posts?page=20#20
In it he posted this link to what Israel could use to justify boarding:
http://www.icrc.org/ihl.nsf/385ec082b509e76c41256739003e636d/7694fe2016f347e1c125641f002d49ce