FYI, just as "my3sons" stated, make sure you glue it in properly and find a good location before permenantly mounting it. The transducer placed in a zip lock baggie full of water is a good way to find a gluing location. Connect the transducer to the unit (just lay cable across deck) and with it...
The Navman Depth displays were always quircky. Lake Lanier has a pretty "hard" bottom, so if you are getting "pings" out of the transducer, you should be geting a good depth reading. You had mentioned reaching over and adjusting or cleaning the transducer? Are you sure you are on the correct...
On that size boat you are better off going to a transom mounted transducer. First, it is simple to install and run the cable. Second, an in hull or shoot thru hull transducer loses 50% of its effective power from penetrating the hull. Yes, you might find a good flat area, but any air pockets...