I'm contemplating upgrading my electronics from something that simply tells me how deep the water is under my boat, to something that helps me identify offshore structure (without having to do it the old school way, and drag the bottom until I hit something).  I've been reading about SI units, and it sounds like the way to go, but I can't figure out how you pinpoint exactly where the structure is.  For example, if I see a tree on the unit, how do I know if the tree is 10 feet to the side of my boat, or 50?

Side Imaging does slant range. It shows the distance from the transducer, not horizontal distance.

Included in the image is the water column on both sides.

You can get an idea by using the range lines.

 

Look through the posted screen shots above.

Do a web search for interpreting side imaging- lots of videos about it.