Test it. That's all there is to it. Most of us that were looking for a thru-hull location tested the transducer on a bag of water in the ski locker.
You put down a ziplock bag of water, set the transducer on the bag upright and put something heavy on top. Or get a friend to hold it.
Then fire up the boat and see how fast you can go before you lose signal. Mine works up to about 45-48mph indicated. And when fishing, it's very accurate and can detect bottom structure as well. Once the location was determined, you put down a small pool of marine epoxy and place it in there carefully as to no get bubbles between the transducer and the epoxy on the hull.
Keep in mind, this is not thru-hull in the true sense. This is shoot-thru-hull. A true thru-hull is drilling through and using epoxy to seal. The transducer is still outside the hull.
Good luck,