If it were me, I’d assume backup paper navigation charts and either a handheld battery powered backup GPS or my smartphone that has an app to parse data from the GPS chip. Could use primarily dead reckoning plus turn the phone on every couple hours to update true location. Most sailors are using paper charts for tricky sections anyways — you just normally have a constant GPS location provided by your navigation system.

There are also lots of established shipping lanes you could find that would be relatively densely traveled and give you periodic feedback for which direction to go. A bit more dangerous due to collisions without lights but as long as your retroreflector is hoisted, big ships should see you clearly on radar. plus you’ll generally be swapping sleep shifts and always have someone manning the helm on the sailboat, though seeing large vessels at night can be somewhat more difficult.