Check to see if every time you turn the unit on if it has the back light on while it is starting up.  I have a Eagle Fishmark 320.  The manual for it lists that it is designed to run on 10 to 15 Volts.  I question that because they are normally used in boats that have 16 Volts across the battery while they are charging and near full charge. Eagle sells a portable kit that uses 8 D cells.  I made a battery holder for 8 C cells and tested the unit at home and it appeared to work ok. I put new batteries in it and went on a canoe trip. On about day 2 it started having trouble finding the bottom and would search for a long time.  On day 3 when the unit was turned on with the back light in full battery sucking mode it would load the battery down and shut its self off.     When I got home I cut the wires to the back light and the unit would work on the batteries from the trip. I have since started using 12 batteries and found the unit works much better.  The unit can now see a split shot and a twisty tail and tell they are 2 items as they are cranked up from the bottom.  I have gotten more than 12 days use out of 12 C batteries.  This spring I have used the unit several days running on 12 AA batteries and it works well.  Although This unit is a very good unit I believe the smaller units are more suited to canoes but check the back lighting on whatever unit you are considering. If it is incandescent and is on while the unit is turning on it’s a battery killer.