Side Imaging transducers have beams that shoot out to the side of the transducer therefore they cannot be epoxied into the hull and work properly.  Down Imaging transducers can be.  However, it is extremely critical that you use a slow cure (24 hours or longer) two part epoxy.  Quick cure epoxy can cause air bubbles to form in the epoxy that will inhibit the transducer signal.  9 out of 10 times that we have complaints about a transducer not shooting through the hull properly tie directly to the epoxy and actual installation of the transducer.