I use a Lowrance X67C on my kayak. It uses a reasonably-sized battery, about the size of a brick. It will go all day and then some. It is color, and excellent at showing fish, structure, depth, and surface temp (although my transducer is mounted inside the hull of my kayak, so I don't rely on the temperature readings too much). It was reasonably priced. I see Lowrance has replaced this unit with a newer model, but it is basically the same. It also has the "flasher mode", which is good for vertical fishing once you've anchored up. If your fly/lure is under you in the the cone of the transducer, you can watch it go up and down in real time (or close to it), and watch fish come up and hit it.
I recommend it. There are other reasonable options available as well.