I fitted one to a boat for an old guy about 40 years ago.
It wasn't my idea and I doubted it would work but he insisted he wanted one.
It had about 4 transducers from recollection.
Needless to say, it made no difference at all, but for about 6 months he was happy.
I'ts a very old idea which seems to do the rounds every 20 years or so when all the people who bought one have died or lost their memory or moved out of the boat business, so they can't tell you that they make no difference.
They work about as well as homeopathic medicine.