A hydraulic lock of 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 hydraulic lock can be controlled through an actuator assembly that is provided between a tiller arm and the watercraft's steered element. One or more cables connect the actuator assembly to other components of the hydraulic lock. A thermal compensator regulates pressure within the hydraulic lock by accommodating changes in temperature related characteristics of the fluid within the hydraulic lock Increases in fluid pressure due to thermal expansion can be mitigated within the thermal compensator by automatically or manually directing a volume of fluid from the hydraulic lock, through a compensating valve assembly, and into a reservoir. Decreases in fluid pressure can be mitigated by directing fluid from the reservoir back into the hydraulic lock.