In a steel boat, the entire hull
becomes the cathode and the aluminum
saildrive becomes the anode (I might have these terms reversed ;-). So the saildrive wastes and not the hull. In fact the saildrive "protects" the hull.
But both are painted and that minimizes wasting and there is a zinc (sometimes magnesium) on the saildrive which is the least noble metal in the combination and it becomes the anode and wastes, not the saildrive.
So maintain the coating and zincs on the saildrive and you should be ok.