Apply the load called "Hydrostatic Pressure" which will create a pressure that is a function of depth below the free surface and a function of gravity and a function of the fluid density. This exactly computes the upward force of the Archimedes effect of buoyancy.

Then you would apply standard earth gravity which creates a downward force on all the mass.

If you are trying to simulate a body floating in the fluid, then you have to adjust the free surface location until the upward force from the hydrostatic pressure is exactly equal to the downward force of gravity.