Depending on hull type, thickness, etc, one option is to mount the transducer you already have inside the hull. If you think it might
work in your hull, you could test it using a temporary oil
bath (e.g. sealant
& short PVC tube). Just keep fingers, etc well away while the transducer is active ...
I mounted a 'transom mount' transducer on my boat (modest displacement epoxy
GRP), and it works very well. I can see the fish
swimming around, and get reliable depth
readings to at least 200 metres. My mount is just bubble-free epoxy
, with the flat face of the transducer 'rolled in' to avoid trapping air under it.
Has worked for me without a glitch for four years ... There are boats it would not work on ... and there may be lots of bubbles in a thick GRP lay-up. All I can say is that mine works very well, and I did not have to drill an extra hole in the hull.