Q:

Water flow's effect on water pressure

So I am not sure if this is a stupid question because I am not someone who specializes in chemistry or physics, but my question is in regards to how flow of water affects pressure of water, meaning, if you went 1000 feet under water and used a device to cause the water to spin in a cylindrical manner, would the water pressure increase, decrease, or stay the same assuming I can read the craft's pressure within the moving water.
My thoughts behind it is water pressure(from my understanding) is just the force of the water pushing on you from all sides, but redirecting water in a direction would change the way it exerts it's force. My thoughts are similar in relation to lets say you fell from 100 feet in the air, the force of your body's fall would likely kill you because all that energy that must come to an immediate stop. Now if you fell that same distance and about 10-15 feet above the ground your force transferred from being going straight down, to being transitioned out of a straight downward fall using a slide that starts perpendicular to the ground and then begins to drastically increase its curve thus allowing the downward force to be diverted so that when you fall you wont feel the full force of the fall by means of that energy diversion. Is the force of water pressure something that can effectively be "abosrbed" or "redirected" by the same means?
Again these are pretty simple ideas and I realize my knowledge is so low that there may be huge holes in my logic, but I am just wondering if it would be possible to manipulate the water around you to decrease water pressure for the purposes of deep see diving, adding less importance to hull strength and freeing up space for people, engine/machinery, and of course the reduced cost of creating that machine.

A:

Water, like all incompressible fluids, obeys Bernoulli's principle.  This means the pressure exerted by a fluid decrease as its velocity increases.  The equation is
$$ \frac{v^2}{2} + gz + \frac{p}{\rho} =C$$
Where $v$ is the velocity of the fluid, $z$ is its altitude above some reference plane, $p$ is the pressure of the fluid, and $\rho$ is the density of the fluid.  This means that if you have one single stream of liquid, the faster it is moving, the less pressure it exerts.
This principle is quite evident in Jacuzzi tubs, which actually do exactly what you want.  In a Jacuzzi tub, the jets are actually connected to two tubes.  The first goes to the motor generating the water flow, and the second is connected to a stand pipe which is open at the top to the air (the top of the tube, of course, must be higher than the level of the water, or water would pour out of it).  The jets are constructed so that the water leaves its tube at a high velocity into the Jacuzzi right past the air inlet.

When the tub starts, the air inlet is actually full of water (all the way up to the water level of the Jacuzzi).  However, once the pump starts moving water, the pressure that water at the tip of the air inlet drops dramatically by Bernoulli's principle.  In fact, this can create a pressure below atmospheric pressure at the tip of the inlet.  This sucks all of the water out of the standing tube, and then starts sucking air into the water stream.  This is what causes the bubbles in a Jacuzzi.
I don't think you're going to have a submersible benefit from this.  The motor requirements to keep this flow going are far nastier than the cost of just building a stronger hull.  However, the theory is sound.  It does work!

