Start w/ a physical examination of the hull and the depth transducer. The best option is w/ the boat dry on the hard.
There are several very important installation instructions for the transducer. It cannot be near other hull protrusions or hull features like chines that will change the laminar water flow.
Another important aspect, also defined in the instructions is the transducer wire routing. A common mistake is to place the transducer wiring the the same trough as other wires, exposing those small transducer signals to electrical noise.