Another thing to consider when using 2d sonar, is that the ping is sent out in a cone shape ... so anything coming into the cone, regardless of from which direction, it's going to appear on the screen as though it came into the cone's front edge of your direction of travel. That would even be true if a fish swam through the cone from right to left, and you were traveling forward. The pings are also not "really" telling you how deep something is, if it's suspended in the water column. It's only telling you how far away from the transducer it is. And it could be anywhere within the confines of the cone ... so, no, you really can't be certain whether something is to the right or left or center of the cone. The unit will read all pings as if they were front & center of your direction of travel.

If your transducer is putting out a 20deg cone angle ... the "circle" of the bottom of the cone is approximately 1/3 as wide (in feet) as the depth of water you're in. So if you're in 20ft of water, the widest (bottom) part of the cone is about 7ft wide. But you also have to take into account that 10 feet down, the cone is only about 3ft wide.
If it happens to have a 60deg cone angle ... then your depth & the bottom circle of the cone would be about the same. (ie - 20ft of water & a 20ft wide circle)

I think that's how it works, anyway. So if any of you depthfinder gurus see a flaw in my description ... then by all means, correct me.

... cp