are you sure it is a Garmin? That model number is a raymarine unit?
I had a quick google to try and help you out. I can't see any option to add a transducer to the unit from raymarine, it isnt even listed as an option......weird.
I can see it is capable of displaying depth information but it may get that from the charts on the unit rather than a transducer.
In theory you can get NMEA 0183 transducers but how it would interact with the unit i'm not sure, the user manual is quite poor TBH. they are not that cheap either for taking a risk perhaps.
I'm sorry i don't know more information but in the absence of anyone knowing better i would be inclined to buy a new chartplotter with stern mounted transducer or a simple depth sounder if you want to keep what you have. that would be the simple option you seek.
a new plotter would be circa £300 upwards depending on what you want/need. Just be careful as some don't come with transducers and or maps, especially the cheaper ones.
Garmin echo 151 with 77/200kHz Transom Mount Transducer