The pressure at depth in seawater is due to an increase in hydrostatic pressure, the force per unit area exerted by a liquid on an object. The deeper you go under the sea, the greater the pressure of the water pushing down on you. The pressure at depth increases by 14.5 PSI for every 33 feet you go down. This nautical depth calculator is used to calculate marine depths (i.e) the pressure at depth in seawater.