Make candidates aware of the pros and cons and differences between GPS and Chart Plotters.
Pre-set Variation & Deviation
True or Magnetic bearings?
Datum offset i.e. OS36 or WGS84?
No accounting for hazards, unlike satnav, a GPS/Chart plotter does not know where hazards are located.
Using a fixed object (buoy) as a waypoint - you may well hit it!
Human error - inputting the wrong position
Technology error - satellite geometry, aerial damage or positioning