This is a very good question. A possible explanation: as the former waiter dude at the recovery site stated, "When they found him he was all puckered up." Perhaps being in the ocean for several days had caused the signs of rope damage to disappear.
Yes, I thought as well about the things like heat and how he could escape and did not.
Plus the sea wearing away marks could be the case.
It is one of my fave episodes like "Suitable for Framing" and The great "Candidate for Crime" with the equally great villain "Jackie Cooper" who later starred in the "Superman films as the Daily Planets" editor.