The NMEA cable should be 2 twisted pairs in a shielded cable. These type cables
should not radiate any significant energy in the AIS frequency band. So I doubt that is the root cause.
A more likely suspect is one or more NMEA2000 devices putting noise onto the cable. This is not uncommon. To find the offending device go to each drop off the NMEA2000 backbone and unplug all the instruments. Then plug
each instrument into the backbone to see which causes the noise to increase. It could be more than one so check them all individually.
If you find an instrument causing problems you can try one of the snap-on ferrites (RadioShack® Snap-Together Ferrite Choke Core - Radioshack
) placed around the NMEA2000 cable close to the offending device. As close to the device connector as you can get it is best. Sometimes two ferrites are better than one and cannot cause any harm.
The AIS VHF
antenna is a more likely entry point of the noise than the AIS unit itself. Take extra notice of any NMEA2000 devices and cables
located close to the AIS antenna.
Good luck and let us know what you find out.