Dudley,
I've had one of these
Humminbird 120 Portable Fish Finders for three or four seasons.
I frequently fish in very shallow waters (less than 5 feet) and it works great. It has an auto-depth mode. I believe the lowest full depth reading is 5 feet. Frequently mine will move to the 15 foot full depth reading.
One of the really nice features, besides being the ultimate in portability, is that it has a side scanning feature that will operate in conjunction with the normal vertical fish finding feature. The transducer can be aimed with a handle under the display holder. As you point it, it will tell you how far away a fish or a school of fish is located.
It will mount on virtually any platform. I use mine in my paddle boat, kayak, canoe and jon boat.
The only complaints I have with it is that it will indicated that you have turned it off, when in fact it stays on. I've learned to disconnect the battery connector when I'm done using it.
The second complaint is that I left mine in an outdoor shed winter before last. The LCD screen has rupture marks that I'm guessing happened when it got extremely cold. However, the blemishes are not real contrasty and they don't affect the rest of the operation.
Because of the way the transducer mounts, it is not suitable for high speed use -- or even medium speed use. My Minn Kota electric motor with 45 lbs., of thrust must be run in speed 1 or 2 of the five available speeds if I am to keep the transducer in the water. Those the the typical speeds I troll at, so it isn't a problem for me. However, it just takes a second to put the entire unit out of the holder and lay it on the floor of the boat or canoe.