I'll assume you're hooking up for DSC/DSE....
So in your hand, as you probably already know, you're holding the ICOM's NMEA0183 comms wires, your Garmin is very much capable AND can communicate/hookup with NMEA0183(as well as NMEA2K) devices such as your Icom VHF.
You require nothing else but to have to wire the two together manually(strip, twist, solder, heat shrink) if you will. See the following info and paying attention to wires functions and not so much colors when hooking up/together.
Icom IC-M323 Manual Online: connections and maintenance, Connections. Connections Q W Q Antenna Connector Connects To A Marine Vhf Antenna Cable's Pl-259 Con- Nector. Caution: Transmitting Without An Antenna May Dam- Age The Transceiver. W Nmea In/Out Leads Brown: Talker B (Data-L)...
www.manualslib.com
https://support.garmin.com/en-US/?faq=va3ugTAr1F4osCzrNKgSA7
Garmin Support Center is where you will find answers to frequently asked questions and resources to help with all of your Garmin products.
support.garmin.com
(Shows a garmin VHF, your ICOM is same difference. Also your Garmin's NMEA0183 hookuo wires are located together with its power connecting wires, ie, same loom.....Check your owners manual for more info. Also also, you'll have to drill down into a few manu items on the garmin to tell it that its comms are on the/in NMEA0183 mode/port)