Depth Finders Find More Than Depth

Another name for a depth finder is a fish finder, because they show you images of what is below the water's surface, up to one hundred feet down. They are not only great for helping you prevent boat damage in unexpected shallow water, but they can show you exactly where to fish, including how deep your lures should be. It might not be shooting fish in a barrel but it isn't that far off.
They use SONAR technology that was created by in just after World War One and used in World War Two, as a way to find submarines. After the war its use was expanded to some large commercial fishing vessels. The cost was so high and the hardware so bulky is would be decades before it would be available for individual consumers. That time came by the mid to late 1980s Since then, they have become even more affordable as well as portable.
Depth finders work by using sound waves that are projected into the water in the shape of a cone, that gets wider and it goes along. When any part of the sound wave hits an object underwater -- a sunken car, a fish, the bottom, weeds, etc., -- it is reflected back to the depth finder.
The sound waves reflection actually reveals the shape of the object, which is then displayed on the screen. The depth of the object is determined by the time it took for the signal to come back.
Some fish finder models are GPS devices as well. GPS = Global Position Satellite. This allows you to mark areas where you find a lot of fish, so you go back another day and see if it is still a favorite spot for the fish to hang out at. It can also be a real life saver, should your boat ever become disabled, you can provide the exact coordinates to rescue personnel.
If you are a real fishing enthusiast, depth finders can pay for themselves, over & over, in terms of the market value of all the additional fish you will capture. The less expensive models begin at less than a hundred dollars. Even the more expensive units in the range of a thousand dollars can pay for themselves, given time.