A tiller-based watercraft steering system is responsive to tiller release to lock a watercraft's steered element in the last commanded position upon tiller release. The system is locked by closing a valve assembly to prevent flow to opposed chamber of a hydraulic lock coupled to the steered element. The tiller preferably comprises an actuator portion which is movable relative to the remainder of the tiller. The actuator portion may, for example, be a grip on an articulating outer end portion of a tiller arm. The valve assembly may be connected to the hydraulic lock to form a combined module. It may be actuated by a mechanical link such as one or more cables leading from the actuator portion of the tiller to the valve assembly. The system may be used as either a manual or a power-assisted steering system.