I ran into a similar problem when i wanted to run my fishfinder without going through the expense of getting a 12 volt deep cycle so here's what i did.
Most fishfinders run on 12 volt battery so you will need a battery pack that you can run in series using 8 1.5V AA batteries. Go to Circuit City and ask them for a battery pack that will hold 8 AA 1.5 volt batteries, it should cost less than $2.00 for the battery pack and what ever 8 AA batteries go for. You'll also need the power wires that go off the battery pack, cost about another $2.00 and come in 2 packs.
You'll want to keep that battery pack water tight so you will have to improvise and make your own container for it.
In the Walmart camping section they sell these yellow soap containers that fit the battery pack perfectly, you'll need to drill a hole in the side of it to feed your power cable from the fishfinder to the battery pack, i siliconed mine. I store mine on my boat bench seat not on the floor, try to find a dry spot for it.
Hope this helps