Good story.

At your blog site you can see the point at which the mooring line failed.

Looks as if the thimble inside the mooring line loop at the bouy end rusted out and in no time at all the line chaffed and finally broke.

Not to ping on the charterer but that problem was identifiable with a simple inspection after tieing up. I've never found it necessary but on inspection if it looks like the thimble is questionable and is rusting out, run a mooring line thorough the bouy ring, double it up and run both ends back to a cleat ....... Or move on to another buoy.