Question

The depth of water behind the Hoover Dam is 220 m. Find the water pressure at the base of this dam.

Answer 1

Pressure under water is called the gauge pressure.
$Pressure= \frac{Force}{Area} \\ F=mg \\ P= \frac{mg}{A}$
Now the next part is difficult to put into an equation because you need the symbol for density, which is ρ, pronounced rho.  I cannot type ρ into the equation so I will use p.
$p= \frac{m}{V}$ (m=mass, V=Volume)
$m=pV \\ P= \frac{pVg}{A} \\ V=Ah$
A=Area of the base of the object, or cylinder of water above the object, h= height of object
$P= \frac{p(Ah)g}{A}$
The "A"s cancel, leaving us with
$P=pgh$

You are only looking for the water pressure, so you would not be concerned with atmospheric pressure on top of that.

$P_{gauge} =p_{liquid} g h_{object}$

The density of water is $1000 \frac{kg}{ m^{3} }$
Use 1000 for ρ
Gravity is 9.8
Height is 220

$P=pgh \\ P=1000*9.8*220 \\ P=2.16*10^{6} Pa$
(Pa is Pascals, the unit for pressure)