EarthNC - Nautical Data for a Digital World

All answer your question in 2 parts:
1. Offline vs Online with EarthNC ChartS:
EarthNC Plus includes two chart modes - ‘Online’ and ‘Offline’. In online mode, Points-of-Interest and special layers are sourced from EarthNC.com to provide direct access to new and real-time content.

In ‘Offline’ mode, all chart and POI data is drawn from the local hard drive.

2. Online vs Offline for Google Earth
Online use is the ‘normal’ use for Google Earth. Offline, Google Earth will ‘remember’ up to 2GB of the most previously viewed imagery. This makes previewing your boating trip prior to connecting important so that Google Earth will store up the images for your local area.

- Virgil