Explain what it means when we say that Earth's oceans are in hydrostatic equilibrium. Now suppose you are a scuba diver. Would you expect the pressure to increase or decrease as you dive below the surface to a depth of 200 feet? Why?

Hydrostatics put simply means fluids at rest (hydro-fluid, static-rest)

Oceans are basically bodies of fluid (water),

and as such they are in hydrostatic equilibrium or hydrostatic balance when they are at rest, or when the velocity of flow in them are constant over a given time. Hydrostatic equilibrium usually occurs when external forces acting on the ocean bodies balance each other out. An example is the force of gravity being balanced by a pressure-gradient force.

The pressure felt by a body in a fluid (water) is given by the formula P = ρgh

Where ρ is the density of the fluid

g is acceleration due to gravity

h is the depth of the body in the fluid.

ρg is constant for water and as a result the pressure you will feel in an ocean will depend on how far below the surface you are, h. The lower inside the water you are the higher the pressure you feel.