What type depth finder do you have?
Mike is on vacation so I''ll answer for him:
Yes, he (actually - "we") took his transom mount transducer and epoxied to the inside of the hull below the water line. This was an Apelco fishfinder.We tested it by placing it in a pool of water in the same spot first. It read depth just fine glued in place like that until the unit eventually failed.
My SR Mariner Thru-Hull Transducer is also epoxied to the inside of my hull and this also works fine although I suspect that the max. depth it can read has been limited. I''m more interested in how shallow the water is than how deep so this works fine for me. I had guidance from SR Mariner on how to do this. Its a common procedure.
Upside: - one less hole in the boat
Downside: possible decrease in max. depth
transducer is "permenantly" mounted