The NMEA 0183
physical layer is based on EIA-422. Is your serial
converter an RS-232 to USB converter? NMEA devices can sometimes drive a single
RS-232 connection (even though it isn't really in the physical layer standard) but if you have existing true NMEA 0183
devices on the circuit then you would need an RS-422/485 to USB adapter.
If that is what you already have then it could be an electrical
problem with the comms circuit (probably a grounding issue with the new device). You could put in any RS-422/485 isolator/repeater to test out that concern. Actisense (as linked above) has them, or you can get non-marine industrial control versions generally for less money
(and some of them are conformal coated and so really no problem with the marine