You know you can buy any unit and make it portable, right?
http://www.ofncommunity.com/forums/index.php?showtopic=31123
I have an Eagle 168 that I got for $60 (or around that price, new) 3 years ago. Got a motorcycle battery, stuck it in a milk crate, put a wooden lid on it and mounted the unit to the wood. The transducer is mounted to a stick that is then clamped on to the back of the boat. Save some money and D.I.Y.