Moving Water Creates a Vacuum? I have a question about Bernoulli's principle as it pertains to water. A scuba diver instructor says that in order to back fin (swim backwards), you just need to displace water behind you and that creates a vacuum to suck you backwards.
He always uses the analogy of the breaststroke. He says that the arm movements of the breaststroke displaces water in front of the swimmer and that creates a vacuum and sucks the swimmer forward. Of course, this "vacuum" is not the only means of propulsion in the water of a swimmer, but does what he says have any legitimacy?
 A: Unless you're pulling hard enough to cause cavitation, which I rather doubt, he's wrong.  The primary reason for propulsion is good old action-reaction.  You push water backwards; momentum conservation pushes your body forwards.
To the extent that water "rushes in" to the displaced area in front of you, that has little to no effect.  See the famous Feynman  "reverse water sprinkler" experiment.
A: I will play devil’s advocate and argue that there’s a colloquial sense in which he’s not entirely wrong.
Say you pull your fin toward yourself while keeping it fairly perpendicular to its direction of motion. The average pressure on the upper surface of the fin increases and that on the lower surface decreases, resulting in a net force that accelerates you in the direction your feet are pointing (“backward”).
I think in an informal sense it’s acceptable to refer to pressures lower than ambient as experiencing some degree of “vacuum.” In this sense, part of the swimming force could be attributed to vacuum.
Side note: this is unrelated to Bernoulli’s principle.
A: Your instructor is wrong.
At the speeds you are swimming (way less than the speed of sound in water) water is incompressible. This means that the density of the water is constant everywhere.
Propulsion via swimming is physically much more correct described akin to a rocket engine: As the fluid through which you are propelling yourself is viscous, you exert a force on the fluid, and the counterforce to that, communicated via viscosity, propells you forward.
The Bernoulli equation is insufficient to explain this propulsion mechanism, as it is missing the viscosity term.
