FWIW, I have a Lowrance mark 4 Fishfinder/GPS unit, and I mounted the factory transom mount transducer inside the solid hull forward of our swing keel. I used 3M 4200 and it works well so far. The 4200 was what I had lying around, and I needed to get it working. I saw it as a good temp way to do it.
It appears to be reading accurately based on known soundings, But I plan on confirming it next year with a lead line. The marked channel into my marina, once past the breakwater, is very shallow in some areas. The low water this year, and the wind induced "tides", make it even more important.
Eventually I may remount it with epoxy like the manufacturer recommends.
By all accounts, removing the transducer from epoxy takes a quick rap with a hammer, without damage.