When I was looking at a boat to purchase, the owner had a Haines 600F. He didn't know anything about sounders as he mounted it far right just off the swim platform, but because he didn't know if that was a good spot or not, he just used sikaflex and it stuck. I was surprised that was strong enough to hold it while testing the boat out at speed, but it worked. So no screw holes.
Just something to think about if you want to try different spots without holes.