You probably know this already (but just in case): Any shoot-through-the-hull transducer needs to be shooting through something "solid." On a fiberglass boat the thing to watch out for is coring. It won't shoot through coring.
So if you have a cored hull you need to either find a place without coring (sometimes there is one or two), or you can cut out a circle of inner skin and core (larger than what you need to allow room for the next step), then fillet around the perimeter (between outer skin and new core edge), and then glass over the whole shebang (so the new glass overlaps up onto the old inner skin, which you have sanded/cleaned).
Now you can glue down the transducer (or it's surround in the case of the P-79) and it will be shooting through solid glass (plus your core will be closed out properly).
If this isn't your problem, then nevermind.
I have a shoot-through-the-hull transducer (not a P-79 because my uncored spot was flat enough that I could just glue a transducer to the hull without needing the "angle changing" feature of the P-79). It works fine up to at least 28 knots.
********
Side note:
In the case of the P-79, what you glue the surround to the hull with just needs to be liquid proof, so the liquid will stay in the surround. If you are gluing a transducer straight to the hull then it's best to use epoxy, as it conducts better than caulk, silicone, etc. (not that I would use silicone just on principle anyway - hate that stuff).
