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Without tearing it apart, probably what's known as a choke for noise reduction. Usually an iron ring with a couple of loops of the power wires through it.
They're primary purpose is to suppress the noise coming from your electronics. Fish finders are very noisy devices and the cables act like an antenna for the signals they carry. A ferrite "chokes" the RFI transmission which is why you find the beads at the ends of the cables closet to the unit.