All ships in the harbor should be randomly pinging their active sonar at full power. And if you see any indications of divers actually in the water near your ship, take a plastic bottle and fill it part way with dry ice. Tie the bottle to a heavy weight. Add a bit of water, then seal the cap tight and drop it over the side. *BOOM* Won't do any structural damage, but the shockwave will rupture the eardrums of anyone down there.
from the article: With multiple ships disabled, Gaza flotilla organizers regroup in Greece