You can get any kind of fancy chain/cable you want, but if someone knows what they're doing you're screwed. Cables/chains/locks/etc are pretty easy to deal with using cutters/sawzall/torch, which is why you want insurance. As long as it's not something the local 12-year-olds can cut with their safety scissors, you'll be fine.
In my experience a good 3/8" or 1/2" chain will last for years and years submerged, buried, etc but I don't know about salt water.