Hi Joffo,
I'm no expert (I'll leave that for Scotar or the moose) but I have a simrad and HDS carbon networked with NMEA 2k and ethernet.
I think the ethernet allows the sharing of a transducer only between units and the NMEA2K allows for all other data (GPS marks, charts etc).
Regarding the power I am pretty sure you will still need power to the unit separate to the NMEA2K through the normal power cable. They different fuse sizes anyway from memory.
Again no expert... Cheers