Budget sounders refect a signal from air, metal and solid material. Mackerel have no swim bladder so they will not return an echo as an example, you will however get a good echo from a ling.
You need to be spending mega dosh to get reliable indications of fish, £3K plus minimum.
The sounders the majority of RSA Boat anglers use are more use in showing depth and bottom make up. Learn how the sounder shows the type of bottom you are fishing over, couple that to the depth and mix that with the knowledge you have on which type of bottom each species of fish prefer and you are on a winner.
Let's have a look at bass, they are a predatory fish as evidenced by their speed and the position of their eyes and sensory organs. When they are switched on to sandeel they will be hunting in areas where the sandeel live. So we are looking for sand banks on the sounder. Now if there is an area where there is a decent current, sandeel like loads of oxygen in the water, and that current gets funnelled between two large underwater rocks. Again if you can interpret the sounder display you will be able to tell sand from rock. The sandeel will be swept though this area by the current. Bass will be in the slack water behind the rock using their good sight to spot the eels being swept through. They will be in the slack not the current because it takes less energy to stay put and let the food come to them.
So now you need to know how deep the 'grey line' is over sand compared to rock, you need to read the sea surface to see where the rocks are and the current is being funneled. Your sounder is never going to show you the odd bass or 2, it may just show a shoal. But shoals are a rare commodity, you will get more fish out of the right area without ever seeing a sign of them individually on your sounder.
I have a little chuckle to my self, no disrespect intended, when a company shows a picture of their sounder showing your end tackle hapily fishing 36.3 feet below the boat. I am of course aware that my tackle is in the water and do not need £400 quids worth of electronics to tell me that, it was me that put the gear there in the first place, what I am more interested in is where the fish are.
Brings me back to the start of my post, most budget sounders are great in identifying the bottom, the depth and structures, learn to interpret that information and feed it into the procesor that you naturally have to work out where the fish are, you will have much more fish than relying on your sounder to show you where the fish are and then watching your tackle to decend to the depths and stopping it right in font of the fish's nose.