I have to say that a fish finder is a fish finder! As a professional guide, the 2D sonar is vital to locating fish. The frequency used by most manufacturers is 83 kHz. This provides a cone that is 1:1 meaning in 30 feet of water, your cone will be 30 feet wide on the bottom. The sonar screen will show fish in the form of arches. Arches have size, shape, and color. All 3 determine what type of fish it is. While some education and practice, you can determine the species under you. Sonar is not great at showing structure. Side imaging and down imaging shows the best structure, but isn't great for showing fish. A Humminbird Helix 5 or 7 would be great on a kayak. Don't mount the transducer in the hull, mount it outside so you get the best readings.
Capt. Craig Copeland
Licensed by the US Coast Guard
2013 Nautic Star 2110 Shallow Bay Boat
Humminbird HELIX 12 CHIRP MEGA SI GPS G2N
Nautic Star Boats Pro Staff (nauticstarboats.com)
Redneck Fish'n Jigs Pro Staff (redneckfishn.com)