My experience has been that modern depthsounders usually can't use the old transducers. However, it's worth a try if you can figure out make and model of what you have.
I have no experience of handheld depthsounders, but concur that, when you really want to know your depth
, you don't have spare hands or attention to look at an instrument you're holding in the water.
If your hull
is solid glass below the waterline, you can mount a new transducer inside the boat against the hull and get good results without putting a new hole in the boat. Find a spot away from the keel
and turbulence from the prop. Sometimes you can find a transducer purpose built for inside mounting. If not, you can glass a short section of PVC pipe against the hull, fill with water or antifreeze
and mount the transducer there.