The fractional compression of a fluid under pressure is given by $\frac{\Delta V}{V} = \frac{P} {B}$, where $P$ is the pressure and $B$ is the bulk modulus. The pressure at the bottom of an ocean is given by $P = \rho g h$, where $\rho$ is the density of the fluid, $g$ is the acceleration due to gravity, and $h$ is the depth.