Someone asked about the in-hull puck transducers over on malibucrew.com, and here is what I stated about my install:
I ended up mounting my puck under the transmission of my direct drive and have not had any issues with it.
I sanded the 'bumps' in the fiberglass smooth and did a trial run with silicone before permanently gluing it down with slow cure epoxy. It worked just fine, I came home and epoxied it in place.
If you go this route, here are a few things to consider:
1) Constant water and no air bubbles will give you your best and most accurate readings
2) when you are determining the location for the in-hull transducer, you want to locate it in a part of the hull that remains in the water, even at high speeds, i.e. towards the stern, near the keel/as close to the center line of the hull as possible.
3) Place the in hull transducer at least 2-3 feet after or just before anything that would cause turbulence in the water flow i.e. raw water pick-ups, paddle wheel sensors, thru hull ballast fittings, stakes. Even if you keep the transducer 2-3 behind or just ahead of these things, try to offset it as well so it isn't sending signals from the same linear plane.
4) Test your location before you permanently glue the transducer in place
5) When you first use silicone for testing and epoxy later on, make sure you don't just stick the transducer to the floor in a vertical direction. Get it in place, then work it around in a circular motion. This will help eliminate air bubbles in the adhesive, which, again, will cause poor/inaccurate readings
6) When you finally do epoxy the transducer in place, be sure to use a SLOW CURE epoxy (8-12hour) The fast cure epoxies create heat as part of the chemical reaction, which can damage the sensitive electronics inside the transducer.