Personally, I would look into the boat’s steering mechanism and see what could be added to control the rudder or propeller/outboard motor.
Electric linear actuators will not move without the electric current applied, would prevent any manual turning of the wheel, and would scare me with safety concerns to not want to be involved with it.
A stepper motor with gear reduction is more my preference. The output shaft could be connected to a rack-and-pinion system, a gear/pulley shaft, or a crank-and-connecting-rod. When the stepper motor has power, it can hold the steering wheel in position; when the electric power to the stepper motor is turned off, the steering wheel should turn manually nearly the same as before the modification.