That is correct! This was a floating Dock in the Muskogee Harbor on the Arkansas River. The Square Blocks are Concrete Anchors and if you look close can see the stell cables attached to them. One thing that is interesting is the bottom anchor that dragged you can see a depression like where it was dropped into the water and made a crater then was dragged under the dock. On the Right you can see the big dock and if you look there are some fish suspended under neath of the dock. There are a few larger fish on the left side and some bait supsended in the water column.
Hope this helps your learn more about your Humminbird SIde Imaging unit.