I am very familiar with how depth finder works. I have two on my boat. They are great on the deep lake I fish on in Canada. I have actually watched a fish come up and grab my drop shot on the screen of my depth finder. I only use my kayak in the shallow rivers, less than 3' in most places, near my house. No need for a depth finder there. I can see, with my eyes and polarized sun glasses, the whole width of the river and the water is clear enough I can see any structure. Even where it is deep enough for me to use my boat the average depth is 4 to 5'. There are occasional deep holes of 15 - 20'. I know where they are and usually only fish them in the winter or the dog days of summer.
I just stated that my kayak fishing is limited to very shallow water and have no need for a depth finder on it. I never said no one needed a depth finder on their kayak.
12-14' is not "shallow" if I am in 14' of water I much rather fish out of my comfortable boat than a kayak. The Kayak is only for places I cannot get the boat. Unfortunately the river I fish a lot has changed. So I had to buy a kayak to get where the fish are.