The image on the right is exactly what you see on your Sidescan unit.
The boat itself can be imagined to be at the top of the screen, in the centre. The light coloured vertical bar is the boats track - so you can see that the rockpile is now behind the boat.
The transducer produces a wafer thin fan of sound which extends from straight down under the boat, to about 5 degrees from horizontal on both sides. Because it looks straight down you can see the water column, just as you can in the standard sonar image, except in the sidescan image it extends right and left instead of down, so the black area in the centre is the water under the boat.
The transducer receives echos from this fan of sound, and simply plots the return, together with an indication of its signal strength (brightness) on the screen in a line extending left and right from the boats position at the top of the screen. This line of returns is persisted and moved down the screen at a predetermined scroll speed, as the boat moves forwards the image builds up as the thousands of lines of returns move down the screen.
To help get your head around what you are looking at - you are looking straight down at the top of objects below the boat, and you are looking at the sides of the objects that are 85 degrees. It naturally follows that you are looking at 45 degrees at something which is half way between the two.
This is no different to flying above a city, where you are looking at the sides of buildings in the distance, and the roofs of buildings straight below.
Sidescan gives you no idea of the depth of water at any position other than directly under the boat. However, by measuring the sonar shadows of objects, or even fish, you can get a good idea of their height and position in the water column off to the sides of the boat. This is easily done in software such as Reefmaster.
By saving your sidescan logs it is very easy to quickly build up a complete picture of areas of the lakebed, enabling you to target exactly the right places.