Question

What is the external-internal pressure difference when the diver's lungs are at a depth of 6.1m (about 20ft)? assume that the diver is in fresh water. (a scuba diver breathing from compressed air tanks can operate at greater depths than can a snorkeler, since the pressure of the air inside the scuba diver's lungs increases to match the external pressure of the water.)?

Answer 1

Snorkel connects the diver's lungs with the air above the water. Because of this, the pressure inside the diver's lungs is the same as the atmospheric pressure.
The static pressure of a fluid is given with this equation:
$P=\rho gh$
The density of water is 1000kg/m^3.
$P=1000\cdot 9.81 \cdot 6.1=59841 Pa$
Standart atmospheric pressure is 101325Pa. 
The difference is:
$\Delta P=101325-59841=41484 Pa$