The sonar's transducer is mounted on or through the hull, where it sends sound pulses down and listens for the echoes. In this video I install the Raymarine RV-320 transducers for my Axiom Pro, which gives both down-looking and side-scan sonar, and attach the anodes to the hull.

For a smaller boat, the Raymarine Wi-Fish is a simpler install: its transducer goes on the transom, and it shows the sonar on a phone or tablet instead of a chartplotter.