I would go with what catfish said. The Humminbird PiranhaMAX portable series is good. Comes with a battery, transducer, and a case with makes the unit very portable. I have the 160, cheapest model for my yak. Does the job, though looking to upgrade. Can just swap out the unit. Think a Lowrance also will fit in the case. This unit might be a good start for you:
http://www.humminbird.com/Products/571-HD-DI-Portable/
