I can't answer your question, because I've been too much of a cheap$kate to buy a sonar unit.

I have thought about sonar for my canoe, but unless the canoe has a motor on it, paddling around a lake looking straight down at the bottom is awfully hard work. To me, it doesn't seem to promise much benefit when the water is shallow like for areas that would be interesting to a fly angler. However, there is one fairly new technology that finally makes sense to me for a canoe. You can scan everything in casting range without moving, and you don't need to look through your hull. The transducer can be mounted to the side of the canoe.

Of course, with the cost of this new technology, you might be better off buying yourself a fishing boat and keeping the canoe for floating down rivers. Here it is: Humminbird 360.