I have $46 million in equipment I am responsible for. I have had some of it chained, but switched to cable. Chain and bolt cutters are a quick cut. However, cable is not all that easy with bolt cutters and there is always a few strans that need more cutting on them. To me, that slows them down and my provide the extra time to expose them and get them caught in the act. Like I said, chain cuts easy, and you can be outta there.
If it's going to be in water, I would get rubber coated and at least half inch. Of course you are going to use a lock to secure this, and locks cut just as easy as chain, unless you get a hardcore lock.