That "free surface" field probably should never have been added to the software. The intent was to allow you to specify the free surface, IF you did not specify global coordinates for your model (therefore your first node would be at 0,0,0).

The problem is that if you have wind in the same job, the free surface MUST be at zero because that is where the wind datum is.

If you need to consider different tides, copy the wave vector and change the water depth.