If you have a watch that relies on a radio signal for accurate timekeeping, lack of this signal may be worked around (on land) by looking up in tables the time of sunrise or sunset and comparing this with the time as told by the watch.

The watch may then be corrected, or if it has no facility for so doing, simply remember that it be say 6 minutes slow.

In time you may find that such a watch looses say 30 seconds a week and this correction may be applied if making a sea voyage.

If making an ocean crossing after TSHTF, then it would be sensible to carry a number of timepieces, and a record of how many seconds a week each one gains or looses.
The time of day for navigational purposes may be taken as the average of several timepieces, after correcting for known errors.

A top quality mechanical watch should keep excellent time, and several should be on board an ocean going vessel.

If TSHTF and you don't have a first quality mechanical watch, then remember that in the early stages of an emergency that you should be able to buy one cheaply as people sell or pawn them for other supplies.