I use a humminbird 190c, love it.
Works great for depth, temperature and contour, which is what you need to focus on using it for.
Before you even buy your FF, make sure you take the kayak out a few times, that way you know where you want to mount the head unit and the transponder. I have my mounted on an arm with a ram ball on the bow of the kayak, I wanted the transponder in the water and did not want to mess with any scupper whole mounts. I like the arm, it puts the cone in front of my kayak which is where I wanted it, and I have a system to swing it up and out of the way if I get close to the bottom.
Getting a FF is a good idea, but I highly recommend spending a few hours in the kayak before ever rigging it up.