WHAT MAKES A FISH FINDER GOOD FOR A KAYAK
Fish finders have three main components:
Display - this is the actual unit - the screen and buttons
Transducer - this is the part that sends signals into the water and receives them. After receiving, it sends the signals to the display, which translates it into images we can understand
Power source - On a regular boat, this is your standard 12V marine battery.
Display: In choosing a kayak fish finder, the display size is the most important thing to consider. Anything larger than 5-6 inches will become too clunky and just get in your way - especially since kayaks are not very wide.
Something very big will also be very heavy, so it gets trickier to mount, too - if you use a ram mount on one side of the kayak to mount the display, you are adding 5-10 pounds of weight(from 7 inch plus units).
Bottom line - go for something less than 6 inches.
Transducer: All fish finders that we carry come with transom mount transducers out of the box. You can either mount it inside the hull, where it will shoot through the plastic without a problem, or you can run it through a pipe though the scupper hole. Lowrance also makes a suction cup mount that you can use to stick it on the side/back of your kayak.
Power Source: When you install a fish finder on your kayak, you'll have to set up a power source, too. This can be a standard marine battery put in a waterproof(very important!) box - then you can run the wires from the battery to the transducer and display. If you don't want to do all of that DIY work, you can opt for a portable fish finder, which have waterproof battery packs built into the case. Just pop in some store bought cells and you are good to go.