Well yeah on land 750kg seems heavy. In water it's not really. A 50 foot boat/ship could easily snag that and drag it off. A larger ship might not even notice until they brought the anchor up.

As far as the dog piled parent, intelligence services are known to be loony paranoid and do really dumb stuff. Not to mention research and science projects are commonly used as cover. So sometimes the loony paranoia isn't misplaced.