Actually, there are a dozen ways Israel can deter these ships from ever making their way to Gaza without boarding the ship(s) at all. Ensuring that they don’t leave the harbor(s) is the best way.
YLE: Meanwhile an international group of peace activists is planning to take supplies to Gaza by sea as soon as possible, but the Finnish Foreign Ministry warns against taking part in the mission. The Swedish organisation Ship to Gaza told Turun Sanomat on Wednesday that it is readying a vessel to sail to the region. Those taking part are from the US and various European countries. The foreign ministry recommends that Finns stay away from Gaza, citing “an enormous security risk right now”.