Hi everyone and thank you for your input.
Some info: The boat is on the hard and the depth sounder gave readings at some point when pointed either to the ground below or to thick GRP sections of the boat itself (incorrect depths to be exact - probably because it gets readings through the air and not water)
In the manual I used it says that GMI 10 DOES NOT get power through the NMEA2000 network. In fact, I tried both ways, and GMI 10 DID turn on when only powered by the NMEA2000 bus. In either case the results were the same.
I will give the common ground thing a go when I get back to the boat, since it seems the only thing that makes some sense to affect normal operation by cancelling noise on the data lines and the one thing I haven't tried.
Again, thank you all for chiming in.