Allsorts
Why can't the boats be taken or damaged in France so they cannot sail. I suppose that's too easy to consider.
Sky news, June. (I've used this before for the same query)
"One problem now is that many boats don't actually land on the beach. They come round and stay out on the sea and people wade out to get on them.
"Boat launches happen quickly these days. Smugglers have worked out that it's much more efficient to launch the boat elsewhere and bring it round to the beach, allowing your passengers to run into the water and clamber aboard.
And, under maritime law, there's not much the French police can do to get involved.
They're not allowed to enter the water to stop a boat that hasn't asked for help and, well, it's not illegal for migrants to run into the water"
(Sky news has reported that yes, whenever possible boats that come close to the shore are punctured, but its a cat and mouse game - its a long coastline and patrols cant always get there in time)