Fish Finders Blog

Obviously a through hull transducer isnt going to work. There is a transom mount transducer available and the bracket should allow you to get the transducer below the waterline. If the bracket that comes with the transducer doesnt get the transducer below the waterline, then you may need to fabricate a mount for the bracket that came with the transducer.
I’m not sure how long you keep the boat in the water at any given time. This matters for reasons of materials that you use to make the bracket. You can probably get a welder to put something together out of aluminum fairly inexpensively.
You’re probably not the first person who has run into this. Check with your local marine supply or even a pontoon dealership. They may have something out of the box that will work for you.