Good technology costs money.
You might be able to get something for $500, but the screen
will most likely be so small that you won't be happy with it.
Supposedly, Humminbird side-imaging units only work to
depths of 150 feet.
With a conventional sonar, if you got experience with it, you might
be able to tell what a shipwreck looks like, but it would require a lot
of time on the water.
I think your budget is too low, unless you can get a good used 1197c,
which has side-imaging, and a nice big screen. Then again, buying
something used is always extremely risky.