It sounds like Maerin totally spot on answer your question.
But, being unfamiliar with that steering system, my first thought was that it might be an accumulator tank for the fresh water system. So, like sunchaser, I'm wondering what the tank is tied to.
If it is tied to the hydraulics, sounds like Maerin nailed it. If it is tied to freshwater, it could be one of these accumulator tanks:
They are used to provide some buffering so the fresh water pump needed cycle hard on and off so often and so quickly, extending the life of the pump and improving energy efficiency (battery life).