The best method of mounting a thru hull transducer I have found is a good glob of silicone (bubble/air pocket free) and then mash the transducer into it until its in solid contact with the bootom of the hull and then lay a 20 lbs dumb bell on top of it to hold it in place until cured and rock solid. This has worked exceptionally well on a number of Kayaks I have used over the years.

As previously mentioned do be careful about scratching the bottom of the hull where the transducer is mouted, its a great way to start getting false readings.