Marine Goop, Plumber's Goop, or Household Goop I have used them all and they all work just fine for gluing your transducer to the inside of your hull. Just follow the way Perry described and you will be good to go. Epoxy did not work worth a
for me! It delaminated from my hull the first time I went out.
Not only that it was a mess getting it off of the transducer afterwards. Once I did get the epoxy off I glued the transducer down with Goop and it worked perfectly for me for 2 years. I sold my kayak to a guy and as far as I know it is still working fine.
Robert