An autopilot is the extra crew member a trolling boat actually needs. Holding a straight course at 2 mph frees both hands for setting lines and netting fish, and a pilot holding a contour or a waypoint route fishes a breakline more precisely than a helmsman splitting attention six ways.
Systems divide by steering type: hydraulic pilots for hydraulic steering, mechanical drive units for cable-steered boats, and tiller pilots for small outboards. The pump or drive unit, the heading sensor and the control head have to be sized and matched as a system — the product descriptions call out compatibility, and it's worth reading before ordering.
Garmin, Raymarine and SI-TEX anchor this category, and brand-matching to your chartplotter gets you route steering from the screen you already use. For the heading and GPS sensors that feed a pilot, see NMEA Cables & Sensors.