Our biggest "chain security" requirement has usually been keeping the ladder secured to the yard fence over the winter, so a heavy "motorcycle" chain from the big box store has been good enough.
But last week I got educated. Real "security" chain for expensive motorcycles and other things is made by a company called Pewag, here in the US. Their main product is commercial
"traction chains" i.e. tire chains, which get ground down by pavement. So they made a better mousetrap.
Pewag chain is available in up to 12mm diameter. The links are made from BAR stock instead of rod stock, and they are hardened to a Brinell 62, hard enough so you will break ordinary bolt cutters if you try to cut it. Obviously the 12 (or 10) mm chain is damned heavy stuff, and expensive, but for dingy security
and similar uses?
Where have you been all my life?? pewag, Inc. - USA
Available from them in bulk, available from many places in short lengths, some with heavy fabric
covers and similarly robust locks, for motorcycles, but quite suitable for dinghies. No one is cutting this chain without making a scene.