Good that things are in better shape than they look. The 2 issues with welding cable are, the wire is not tinned and the insulation is not as oil resistant as marine grade wire. The problem with the firs issue is corrosion potential and with the second is degradation of the insulation due to oil, diesel, and dirt.
Back in the 60s and 70s nobody expected fiberglass boats to last forever. So, builders didn't take as much care in some of the systems. And for electrical systems, about all they were used for was lighting and starting the engine. Things have changed.