Yes, 19.5 would be the height of the "transducer" off the bottom. Since he is using a Humminbird unit its most likely hull mounted versus a towfish, but he did not say for sure.
One big factor asside from depth in determining the size, if you notice the range is set to 110' and the barge extends from the very first return on the bottom to the very last return. So its length is somewhere around 100'.
I had never considered a bridge or road section, but that makes sense. It could be the girders we are seeing.
Robert