I donít have any experience with DSM, and if cable connections from the transducer to the unit are intact and the display turns on then the fault may be the transducer as you suspect.
I know for sure than in-hull transducers work
, a couple of friends have them (I donít), do a search on CF and you will find relevant threads.
Once you get a new transducer a quick test would be to hang it in the water
from the side and connect it to your ST60, if you see depth you have just confirmed your suspicion.