I WOULD NOT RECOMMEND USING EPOXY OR ANY TYPE OF SILICONE for this application. I have installed units in this manner in my last 4 boats and will say that it works very well. All of the advise above is spot on with the exception of what material to secure the "puck" in place with. Silicone/RTV is designed to be flexible. It will absorb some of the signal that you are trying to shoot thru the hull. Epoxy will hold very solidly, but if you ever need to remove it, it aint gonna happen. I would recommend using polysufite caulk when attaching the puck to inside of the hull. apply nice (1/2") glob to bottom of transducer covering entire puck, making sure there are no bubbles, and press firmly into place. caulk will squeeze out the sides. Let sit for 24 hrs. before use. The caulk will remain somewhat pliable in case you ever need to remove it, but does not exhibit the flexibility of silicone based products. Obviously some have used RTV with success, so it is certainly doable, just not what I would recommend.
hunter Legend 37 Semper Paratus
Formerly - Tartan 34C Yawl