That HAMAS is trying to smuggle arms, rockets from every possible way including the sea is a fact not theoretical possibility. The blockade is there to prevent or at least to reduce the quantity of the arms/rockets. I also admit that some in Israeli establishment figuers see the blocakde as a punishment tool too. But that does not diminish the fact I mentioned above. If you want an immoral blockade like sanctions look at the USA sanctions against CUBA. As an example a ship that visits a Cuban port can not visit any USA port for many month. Is Cuba even remotely a threat to USA now?
Law expanding definition of terrorist activity passes first Knesset reading (Haaretz)