I've done it four times now. On my Columbia 26 I mounted the Humminbird fish finder first near the stern as the instructions recommended (for a powerboat) which ended up to one side because of the motor well and pulled out of the water on a port tack, then ahead of the keel on the same boat, both with epoxy. They worked fine except the stern which had problems when the motor was on or we were heeled.
On our Coronado 34, I temporarily mounted and in hull puck in wax from a toilet bowl ring that worked until I replaced it with a thru-hull.
On our E-38, I mounted the sounder for a Garmin 238 next to the thru-hulls without problem.
For testing I would recommend either the toilet bowl wax or as Garmin phrased it, " personal lubricant". I used the generic equivalent of KY jelly.
Don Wigle
Wiggle Room
Pt. Richmond, CA