This article notes:
The volume of a specific hydraulic oil not only changes due to pressure changes, but also due to changes in temperature. When the temperature increases, the density of the oil decreases, leading to an increase in volume.
Another website also remarks on thermal expansion of hydraulic fluids:
What is thermal expansion and how is it caused?
Thermal expansion is caused in a sealed system by the expansion of the fluid volume due to increased temperature of the fluid.
What fluids are affected?
All fluids...are subject to thermal expansion at varying level..[such as] hydraulic oils...
On this basis, it seems that a boat steering system using hydraulic fluid should probably be vented at the helm pump as a means of allowing for some thermal expansion of the fluid volume in the steering system without an increase an the static pressure of the system.