Price is probably the biggest factor for most people. Get the best unit you can afford.

Unless you go with a fishn' buddy, you're going to have to buy a second transducer. You'll also need to hook up a 12v battery to operate a normal graph. The fishn' buddies run on 'D' batteries and there's no wires.

But, if you want a traditional graph, I recommend a Lowrance. I currently have an X67c on my OE Versa Vessel. It's a small unit, runs on a sealed 12v motorcycle type battery. Color graph with built in temp and all the good stuff. I don't have a single complaint about it. :-) They retail for $299.

Check out some of the pics from the 'float tube/kickboat forum' rigging page.

http://bass-n-tubes.com/gallery/rigging