They could have just asked Jack Bauer
If you ever stumble into an episode of "24" and find that you have to dive under water to escape a hail of terrorist bullets, here's what you need to know to avoid being killed -- of course Jack Bauer would probably have plugged them full of holes before you even got your hair wet!
New Scientist Back Page - Dive, Dive!
In films, a hero often evades bullets by jumping into a river or lake. How far below the surface do they need to dive?
Any object moving through a medium experiences a drag force tending to slow it down. For a denser medium like water, the drag force is much larger than it is in air. Water is 700 times denser than air. The drag force on the bullet scales as the square of the velocity and is also proportional to the surface area of the moving body.
Knowing this, one can set up an equation of motion for the bullet, which gives the distance over which its velocity is considerably reduced. The formula involves the velocity, mass and size of bullet, the density of water, and the drag coefficient.
For a typical bullet with a velocity of 300 metres per second, the depth over which it slows in water is barely a few metres. So a 3-metre dive below the surface is more than adequate