I place sharp stakes in a half circle on both sides of the entrance just below the waterline. They definitely work as I see the leftovers from Greydwarves & Necks on the seabed and have to repair the Sharp Stakes from time to time. Since I started doing this I have never had a sailing craft destroyed. I tend to err on the side of caution though. If I am going to be away from my marina with a vessel moored for more than 3 days and will not be onsite to check the condition of the spikes, I will destroy the ship. If I am planning on exploring an island, I will take two 2 star wolves along with me. I will moor my vessel at the tip of a isthmus or offshore island and place the wolves on land beside the ship. I have never had a vessel destroyed yet when employing this strategy.