The P79 comes with an adjustable mounting housing that you silicone down to the inside of the hull, and adjust so the transducer points vertically, independent of the hull deadrise.  The housing is filled with propylene glygol (non-toxic antifreeze) to transmit the sound waves to the hull.

The rated depth for the P79 is 600 feet.  I routinely track bottom at 3 times that depth. On my Amel it helped to smooth the inside surface of the hull.  I suspect the texture of the surface of the fiberglass scattered the sound waves and reduced it’s sensitivity.

On my old boat, I installed it over the glassed over hole for the old transducer.  So it had a smooth surface inside and out of solid epoxy and glass.