@thetone is this what you mean?
Skimmer.
Transducer that skims the surface of the water. Transom mount
transducers are an example of this type of
transducer, as they are typically installed a little below the hull.
View attachment 175385
If so, it may be possible to stick the unit against the inside of the hull using wax or putty. The issue on a sail boat,
- the transom usually is not flush with the water surface.
- The unit comes out of the water when the boat heels.
Out of the water the unit does not work.