Location Services allows location-dependent apps and websites to use information from cellular, WiFi, and Global Positioning System (GPS)* networks to determine your approximate location. If you're not within a clear line of sight to GPS satellites or your device does not include a GPS receiver it can determine your location using crowd-sourced WiFi and cell tower locations.
The GPS itself does not use data, it merely receives satellite signals independent of cell service. The device actually uses aGPS, meaning it receives 'help' from the cell carrier - but that help uses the voice network, and you aren't charged for it. So, there's no charge for the GPS localization alone.
So, with the Navionics mobile apps, as long as you have previously downloaded the mapping areas to your device (Navionics Boating & Ski apps only), you would not be using any data.  Certain features, such as wind overlay or map overlay or Sync My Data will use cellular data if a WiFi connection is not present.
*Please be aware that some mobile devices do not have an internal GPS receiver.