How did I miss this thread? Yes you can buy something that look a bit like a big corkscrew, you screw into the ground and they are used on building sites to secure valuable equipment with big padlocks!
I have been thinking about the same sort of idea, but using a boat on rivers and such. You only get one shot at life, go for it, mate!
Do you know where to get them from? Are they industrial things or could anyone use them? I would like one for my canoe but I can't see them anywhere on the internet