With hydraulics (In general.. not just steering systems), it should be either on or off, or pressure dependant.
What you guys are describing, is a leaking valve or a faulty pump, which in this case is the steering wheel as the pump. Regardless of the load, the pressure should be holding,.. in your cases it isnt,.. and you want it investigating, your fault is in the ram on the outboard, leaking, and allowing oil to bypass from one side to the other, or your steering wheel pump, doing the same, you keep having to turn the wheel to replace the pressure lost by whatever component is leaking, and compensate for the load provided by the outboard.
This leak will not provide any external signs,.. it will be internal between components so you will see no visual indication from the outside. So assuming your hoses are dry and no oil is evident, and you have enough oil in the system, these are your prime suspects