The January issue of Practical Sailor had an article suitable for a few lines in Post-Apocalyptic Boatbuilding.
It notes that retired climbing ropes (they call them ropes) usually have very little UV damage or wear. And are retired early and often. While they stretch a bit more than our top low stretch lines, they have many uses where stretch is expected or even useful.
There are 5 different kinds of ropes, but I gather that single dynamic are what we want.