Dive down and your ears feel it: the deeper you go, the taller the column of water stacked over you, and the harder it presses. $P = \rho g h$ — read aloud: P equals rho g h. $\rho$ is the Greek letter rho, said “row”, and it is the fluid's density — for water, 1000 kg/m³, a number worth memorizing. $g$ is gravity's pull, $9.8\ \mathrm{m/s^2}$ on this paper, and $h$ is the depth in metres — how much water is stacked overhead. With $g = 9.8\ \mathrm{m/s^2}$, every metre of water above you costs $1000 \times 9.8 \times 1 = 9800\ \mathrm{Pa}$ — call it 9.8 kPa per metre.

Here is the nugget that ties the chapter together: ten metres of water costs about 98 kPa — almost exactly one whole atmosphere. Dive 10 m and you carry DOUBLE the surface squeeze; the ocean matches the entire sky in its first ten metres. Air is just a much, much fluffier fluid — which is the next lesson's whole story.