Israel does not want to have these ships sail for Gaza. This would result in boarding, and the possibility of resistance by the passengers and those who are claimed to want to kill IDF soldiers when they board. As a result of Greece denying these ship's approval to sail and Israel efforts to put pressure on these governments , in effect they are preventing what could end up as another shootout if some passengers did indeed turn violent. So say thank you and go home.
At least 1,000 Israeli Arabs protest death of Palestinian infant in deadly West Bank arson (Haaretz)
from the article: Greece blocks departure of all Gaza-bound ships