Forget the foam block and Marine Goop.....
If you plan shoot through the hull, suggest using Duct Seal to hold the transducer against the hull......in short, use it like modeling clay......pre-clean inside of your hull where you plan to mount the transducer with acetone.....then, using your hands, roll a piece of Duct Seal into a 1/2" thick piece of spaghetti and mold it around the outside of the transducer to the hull.......this will secure the transducer to the hull without any air bubbles between the the transducer and hull.
You can use as much as you desire to hold the transducer to the hull....it will not come loose until you decide to remove the transducer. It's also reusable.
Duct Seal is available from Home Depot in the electrical department
http://www.homedepot.com/p/Gardner-Bend ... /100212441It's cheap and a pug will be enough for 3 installations.....assuming you have multiple kayaks!