On my Garmin 78s I went to Main Menu > Setup > System > Interface > Garmin Spanner.
If you don't have Spanner in the list, use the Garmin WebUpdater utility (free download from Garmin) to update your GPS software
as that feature was added some time ago.
Then connected the USB cable to the computer and on the GPS answered No to the "USB Cable detected. Would you like to go to Mass Storage
Started the OpenCPN programme which immediately put the boat
at my location.
When I start the OpenCPN Dashboard the Position, Compass
and SOG are all active and correct. The Satellite
strength bars are not but up in the top right of OpenCPN I see the 3 green bars icon meaning full strength.
I also tested this on a Garmin eTrex 10 - the new entry level model. In the Interface menu there's only 2 options: Mass Storage
and Garmin. Selected Garmin and it too worked fine - just as above.
I hope this helps.