The bottom of the hull is pretty solid. Here's what you do, with the boat in the water set the transducer in the water and see the depth of the water.(Now you know it works) Look at the bottom of the boat. Make sure the inside location isn't over the jet pump or other items between the inside hull and the bottom. Now scuff the location on the bottom with 400 grit sand paper...wet dry is preferred. Add a little water to fill the area that you are setting the transducer on the bottom of the boat on. This will fill the air gap of the transducer and the hull. Water is pretty dense, so it is like it is fixed to the Hull bottom. Now set the transducer on the water and the bottom of the hull, and see if the transducer "shoots" or picks up the bottom depth. It should be the same as the water you are in. Put the transducer over board to measure the depth than replace it in the location in the hull. It should be the same. If not, try another location. If it reads the same remove the water and dry the spot and use "clear slow dry 2 part epoxy" read the direction, apply and you done. I have done this like 17 times...I do a lot of them for grateful friends...
Karl