The problem will not be the hull but everything else. Can you make rope
from natural fibers without machinary? Wood is the only low tech material that can be worked from scratch with hand tools. Basically look back through the history
of sailing and guess where tech will be down to
If things go down for a long period then we will be back to dugout canoes, mind you the people I live with used to trade
and Hawai using a dugout canoe and woven cedar sails
in the days when they had no metal tools. The chineeze also crossed the pacific in low tech junks before Europeans crossed the atlantic and the Polyneesian voyages as the stuff of mythology. Read Tim Serverin for an eye opener about what low tech boats can do.
All scenarios depend on what tech gets preserved and the ability of the environment
to support people