Based on 3D Navier - Stokes equations for a viscous incompressible fluid a comparison was made between hydrostatic pressure and pressure caused by fluid movement at depth. We consider two variants of exact solutions of Navier - Stokes equations for case of unsteady motion in a large tank when the influence of bounding surfaces can be neglected. Five different types of pressure distribution zones are identified throughout the entire movement area. For the first type zone the pressure significantly exceeds hydrostatic pressure. For the second type zone there is an excess of pressure compared to hydrostatic pressure. For the third type zone the pressure is close to hydrostatic. For the fourth type zone effective pressure differs from hydrostatic pressure to a lesser extent due to deep-sea movement. For the fifth type zone there is a significant decrease in effective pressure from hydrostatic due to deep-sea movement. Relationships have been obtained that make possible to determine the location of each zone at an arbitrary point in time.