Too small of a wire gauge. Run a dedicated positive and negitive marine grade “x gauge” (depends on the amp draw/the actual line length) wire to your unit with the approate inline fuse @ the battery
Yep. Trim the Humminbird power cord to a foot or so and run 10 or 12 gauge from there to the battery. Add a fuse close to the battery, I think it should be a 3-amp but use whatever the owner’s manual says. Use a blade type fuse, not the glass tube type.