Read the Ansys Help page in the Mechanical Application on Hydrostatic Pressure.

There you will see this note:

During a multiple step analysis, tabular data is visible for this boundary condition. This information is read-only but you can use the context menu (right-click) features of the Tabular Data display to activate or deactivate the loading per step.

That means you can deactivate the Hydrostatic Pressure load in Step 1 or any number of steps.