Re: Fishfinder Power
If it's going to be the only thing you'll hook to the battery, one option you might want to look into is one brand, I think Eagle, makes some portable models that come with a base that you put 8 AA batteries in. The unit itself is the exact same thing as a model they offer that your would hook to a 12V battery. That will save you from having to buy a 12V battery. They don't draw much power, so the AA batteries do last quite a while. We have one of these for when we use the neighbors' boat.
There's another brand, I think it's Bottomline, that makes basically one piece fishfinders that you just clamp on the side of the boat. The screen is above the clamp and at the bottom is the transducer. They also run off of alkaline batteries, but I'm not sure what size or how many.
Even if you went with a regular fishfinder, you could probably get some parts from Radioshack to basically make an 8 battery box for either C or D size. It would put out 12V and be enough to power a small fishfinder.
And EXPENSIVE!