You realize that for good mapping you need a transducer capable of switching between transmit and receive very quickly? That a single frequency transducer has a "return time" delay, meaning that you have to wait for the reverbations of the previous pulse to dissipate before you can launch energy again? This is cured with specially designed pseudo-random tone sequences etc? You need own ship motion to make life easy on the computations, and you need to know own ship's position very accurately.
There is big difference between collision avoidance, fish finding, mapping, and submerged target ID on the pond floor. Side Scan comes to mind.,
How Shallow, How Deep? What resolution?
The towed behind the boat, RF linked, low cost fishing sonars have been discussed on EEVBLOG, you might want to start there...
A lesson learned on a six to thirty-six foot deep water reservoir when fishing as a college student, was that a old rotary style Humminbird flasher sonar was great in the vertical plane, unless you where getting echos off the hull in shallow water that bounced back to the floor, and back off the boat hull. However, what happens if you turn that fat cone beam sideways and try to range the massive dam less then 100 feet away, in 36 foot deep water? Well, the range only display showed everything possible except the dam, which from the radar based model in my mind, should have been the dominant path. Instead, lake floor bounces to the dam were shown, and many, many, of them. The radar model failed miserably if I tilted the transducer. Big lake, soft muddy bottom, few obstructions or rocks, and I've seen it drained. It's just muddy plain where I ran my "test". Rip-Rap rocks along the base of the dam did a great job of scattering the direct return.
Own Ship Motion simplifies your problem. Arrays simplify your problem. Having a good navigation fix, priceless.
What do you want to really do? Defining the problem is important.
What I started with on the lake that day was this:
http://www.seekic.com/uploadfile/ic-circuit/2009723225737217.gif , which is a spining disk with a neon as indicator.
That was 35 years ago, consumer sonars have improved greatly.
LM1812 is very obsolete, but had my attention as a kid. The data sheet is worth looking at.
Steve