Check out the transducers that come with each unit. Many Fishfinders
use transom mount version versus the thru hull transducers used most often in sailboats. A transom mount transducer is not a viable option on a sailboat for many reason one of which is that it comes out of the water when heeling.
My boat came with a fishfinder
that had an optional thru hull transducer available. When I enquired on the price of obtaining the thru hull transducer I noticed that for very few dollars more I could buy a whole new depthsounder (not fishfinder