Dr. Bill just sent this possibility:

"The Greeks would have had two choices when
they brought ships to shore. They could beach them
if they had a beach that was just sand and wouldn't
hurt the hull. But if the beach were cobble, they would
have put an anchor out from stern of ship, and then
had a cable running up shore to some place where they
could fasten in. I think the word for cable was 'byssus'.
And our word for deep water 'abyss' meant orignally
'too deep for the anchor rope'."

Thanks, Bill.