It would help if I *knew* what the symptoms are. No PC integration, but the GPSmap 1x2 and depth sounder OK (I'm guessing that this is it). Nothing working. Or, something in the middle.
First thing to check (because I'm old and forget stuff) are the RS232 (Using DB9? -again a guess) pinouts:
Pin 1 Data Carrier Detect
Pin 2 Received Data - Garmin Brown (Com1) Green (Com2)
Pin 3 Transmit Data - Garmin Blue (Com1) White (Com2)
Pin 4 Data Terminal Ready
Pin 5 Signal Ground - Garmin Black - CHECK THIS
Pin 6 Data Set Ready
Pin 7 Request To Send
Pin 8 Clear To Send
Pin 9 Ring Indicator
So, you are using COM2 on the Garmin to connect to the PC. Is there a Setup setting to activate this on the Garmin? What about the COM settings on the PC. I believe that they should be: As an experiment, you may want to try switching from COM2, to COM1 (thus, disabling the GSD20).
Next thing that I would check are the com settings on the PC. NMEA 0183 standard bps settings are 4800 bps; 8 data bits; 1 stop bit; no parity. I have heard of instances where bumping the data rate to 9600 works. You may want to give this a try.
Lastly, you may want to check out a NMEA Multiplexor which should make this whole protocol stuff a lot easier. Here is an example ShipModul Marine Electronics