Last one I did I used duct seal. http://www.pierandsurf.com/fishing-foru ... on-a-kayak
I did mine a bit different that what is shown on the video.
* Used foam to make a wall approx. 1/2" high with a slightly larger inner dimension than the transducer.
* Glued wall in place with GOOP. Allowed to dry overnight.
* Placed duct seal in sun to let it warm up.
* Selected desired amount and kneaded it for several minutes to remove bubbles.
* Put a 1/4" - 3/8" layer of duct seal in the bottom of the "well" that made with the foam.
* Connected transducer to sail mast holder thing-a-ma-bobber (Hobie) with a zip tie that I tightened mostly but not fully. Not yet. I wanted it to slide the transducer down the sail mast holder thing-a-ma-bobber accurately and neatly as the transducer was pressed into the duct seal. Once it was seated I carefully finished tightening the zip tie and snipped off the excess.
It's been working fine for about 1.5 years.
Your mileage may vary.