The many laws govern that kind of situations, (The Geneva conventions and the San Remo Manual) clearly state that nation has the right to enforce a blockade even in high seas (International waters). But let me remind you that Turkey has been killing PKK members by crossing into another sovereign country's borders - Iraq and Syria - for decades. Not even International waters, violating another sovereign state's land.
Palestinian sources: Israel holding secret talks on arrest conditions of hunger-striker (Haaretz)
from the article: Istanbul and Tel Aviv can fix what Ankara and Jerusalem broke