I've had terrible luck with that Humminbird unit...
I installed their model 610 (I think it was). The first transducer didn't read above about 20 MPH. I got a second transducer (too risky trying to remove the first one after epoxying it down) and relocated it. Things improved, but now this year (less than one year later) it doesn't matter which transducer I use - stable readings vanish above 20 MPH.
I'm thinking about moving to a new unit from a different manufacturer, but I'm running out of good places to glue down the transducers.
Caveat emptor!