As a sometime sailor, what Dr. Bill says makes sense and the "byssus/abyss" story is deeply interesting.
Sailors "cast off" when they leave shore. What they are "casting off" is their tethering to the shore.
If they are docked at a pier, they will have lines from the vessel strung fore and aft to pilings on the pier.
Before they can set out again, crew members must leave the vessel momentarily to untether the lines and cast them aboard ship to be used again. Of course, these crew members must leap aboard smartly before the vessel sets sail without them.
So, somewhat ironically, before you can "cast off", you have to "cast on" your tethering lines or cables.
Perhaps 3000 years ago, in the absence of man-made piers, permanent cables were tethered ashore at heavily trafficked landing sites, as Dr. Bill suggests, so that the tethering cables were literally cast off the vessel in preparation for departure, the very opposite of what happens today.
So, it would seem that this seafaring lingo remains tethered to its origins in the past, altho it no longer makes any sense in the present.
Today, sailors cast off lines when they arrive at a dock, and cast on lines when they set sail. But they still say they are "casting off" when they are really casting on.
