I received the N2K interface replacement for the SeaGauge and installed it. Seems to work well. I do not have to get too fussed about this as displaying sensor data in the chartplotter is a backup. The Maretron (yet to arrive) will be the primary as it is more feature rich and leaves the chartplotter for displaying other things (radar and sonar).
I am going to forget about the Furuno chartplotter internal GPS as I can not get it to pickup any satellites (even when out of the boat house). I will wire it to the VHF to get GPS and, hopefully, DSC as well. This means connecting them via NMEA 0183.
The C-MAP I purchased seems to be 2009 and Furuno is warning me that it is out of date. I have to figure that out as well.