It fails the first test of legality i.e. naval blockade is part of the Laws of War and, therefore, there must be an armed conflict before a navy can legally blockade ANYTHING. When that flotilla set sail for Gaza there was no armed conflict; operation Cast Lead was already over. Israel can no more blockade Gaza because it doesn't like Hamas than it can blockade Tyre because it doesn't like Hezbollah.
EU's Tusk warns Turkey to cut refugee flow (Reuters)
from the article: U.S. scrambling to prevent Israel-Turkey ties from worsening