Larger boats commonly use hydraulic steering. For an autopilot, an hydraulic pump is used. These pumps usually consume about twice the power of an electro-mechanical drive because of friction in the seals.
The hydraulic motors tested draw 10-12 amps while running, and up to 30 amps stalling. This means they need a higher power controller to handles these currents efficiently.
It is also recommended to use rudder feedback, or end of travel switches to avoid stress on the hydraulic pump.