If you can locate an ST60 Multi-display unit, it will be your ticket. It will connect to your ST60 depth via a SeaTalk cable, and the rear of the Multi has NMEA IN/OUT ports. It is capable of SeaTalk/NMEA interface. No other wiring is needed, since it's powered via the SeaTalk cable. You can connect the output to your Garmin and port the depth to it, and if you connect the Garmin's NMEA OUT to the ST60 Multi IN port, it will display the data ported out of the Garmin. You'll probably have to go into the Garmin's configuration and choose the NMEA sentences to be turned on for the port you're outputting. AFAIK
, the Garmin has 2 NMEA output ports, you'll have to check the manual for color coding on those ports. The ST60 will display most of the NMEA sentences the Garmin can port out, some of the newer ones (ex. AIS, DSC), nor the Garmin proprietary sentences it won't recognise, but it would give you another display for NMEA data. They're a handy item.
As far as the NMEA for the ST60 color coding, it doesn't matter, the ports are clearly marked IN/OUT/+/- so color is your choice. You should use shielded twisted pair cable, or CAT5E or CAT6 works fine, too. The twisted pair is the important part to prevent cross talk of the NMEA.