Throughout the 1950s, the first sonar units built specifically for fishing made their appearance. They were expensive. They were difficult to use. They were clunky.
They weren’t popular.
In fact, a 1958 Lowrance Fish LO-K-TOR would run you as much as a Browning shotgun.
However, as time passed the “Red Box” eventually became the famous (portable) “Green Box”, which worked well enough to become commercially successful. Soon after, others followed.
Before the introduction of sonar, the surface of the water stood between fishermen and the world below. The world inhabited by the fish they were chasing. If the water was especially clear, anglers could see some objects below the surface, but in murky water the nuances below remained a mystery, and the same held true for deep water.
Early units were simply depth indicators, but improvements quickly appeared. Not to mention as anglers learned what the different signals (or signal strength) meant, they were able to identify bottom composition: mud, rocks, sand, based on what their units displayed.
This allowed bass fishermen that had primarily been limited to visible cover & structure, to confidently move into deeper water.
It also helped fishermen working the depths for other kinds of fish as well, by making underwater cover visible.
These humble beginnings were the building blocks upon which today’s super-advanced electronics were developed.