Everything is of course possible for a trained developer.
Next is the to get data and have knowledge of its structure. The e.g. a Garmin Fishfinder
is developed by engineers knowing exactly the structure of there's own devices. To find out these facts would be a reversed engineering like the radar
plugins. Garmin would likely not share there's knowledge.
It's up to anyone to develop a plugin for that. But I'd say this has not much to do with navigation
that is the main goal for OCPN.
You must buy the echo sounder to get the data. Why not use there's device for the fish
picture as well. instead of reinvent the wheel
on system where not so much money
can be saved? At least compared to a radar MFD system.
data figures could though be transferred to OCPN for a recent depth
info in the inbound Dashboard or other plugins.
The above is just my humble opinion. Nothing else.