Good idea. you can make it cheaper, yet, by saving the cost of the bucket and the cement. And the labor of mixing the cement. You don't need it.
A couple years ago I put four tie downs in my dirt driveway to secure my fishing boat ( Contender) to the ground during hurricanes. Very similar to your idea.
But I figured out that if I just stuck a bunch of 10 inch or so pieces of steel rebar through the chain links, and packed the dirt back down, it was pretty danged solid. You sure wouldn't pull it out. It looked something like this:
For a temporary ground anchor you could take with you in the boat, how bout one of those screw in, helical anchors you can buy at hardware stores?
My experience with most hoodlum type crooks is that they are not going to work very hard to try to steal something. If it's not easy, they will move on to something that is. They don't carry lockpicks or bolt cutters with them, generally.
Island life in the Devil's Triangle: http://2gringos.blogspot.com