Well, be suprised all you want.
I was a marine electronics installer for 4 or 5 years and dealt with it quite often.
Plus I have run as many as several hundred boats in a year while working for a boat dealership.
There are times in shallow water where you can clearly see a break in the water column, the digital number on the screen is intermittent and so is the bottom.
Frequency change or isolation sometimes helps if incorrect and so does filtering and gain. But in depths less than 15 feet or so true water depth, and a few less because of ducer placement, there are huge number of complaints about losing bottom for the reasons that others have explained to me including representatives from all the major marine electronics corps.
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