Some "puck" transducers will work without the need to drill a thru-hull. However, the problem with the transom mount is twofold; 1) Heeling of the boat can and will cause an inaccurate read & 2) Turbulence from rudder will do the same.
I'm sure you can find people who will say they've mounted their transducers in all kinds of places, but if you want reliability, then the thru-hull is the way to go. Always check ads, if you're looking at used, and make sure both the transducer and the power chords are included. Aftermarket, those can be more expensive than the unit itself.