Calculate hydrostatic force on a flat panel

I have a vessel's hull (submerged part only) discretized by flat panels. Each flat panel is defined by 4 nodes in 3D. The origin of the coordinate system is inside the hull in the water plane (all Z-coordinates are negative). The nodes are defined in a regular order, providing a path along the boundary of the panel.

How it is possible to calculate the hydrostatic force on such panels? The increase of the hydrostatic pressure with decreasing Z-coordinate must be considered.

Thanks in advance, Michael