Beerman -
I have a Hummingbird Piranha 3. I bought it, basically because it was on sale for $70. It is compact, and works well. It shows the depth and water temp, but I have it mounted inside of the boat, so not sure how well the hull material insulates, so that probably wouldn't be all too important of a feature. I have never used a different model, and have only seen the very high end ones, on charterboats before, so I have nothing to compare it to. It would be nice to have one with the topographic view, gps, and maps built in, but being in a kayak, you really aren't going to have time to explore a whole lake, if you want to fish too.
I have it attached to a 12 volt cordless drill battery (Radio Shack - female Quick Disconnect connectors). If I charge the battery, I get more than a full days use out of the battery, just remember to charge... I have found that not having the transducer mounted (glued) works better for me. You will need a rag, and a jar of petroleum jelly though. I had attempted to exopy it a few times, but pops off too easily. You hull is probably a lot more stiff, due to it being a sit on top, so might be worth a shot. I'd have the epoxy pop, for just moving the boat from my roof to the ground.
Here's the unit that I have. Not the best, but does what I want it to do. And yes, I have caught fish because of it. Especially if you are looking for those quick drops and sunken structure that they show on those treasure maps.
http://www.marine.com/product_info.php?ref=68&products_id=1670&affiliate_banner_id=1