Is a causal relationship implied by Newton's 2nd Law? Throughout my time learning physics I have been imbued with the notion that forces cause accelerations, period. Accelerations don't cause forces, and they aren't merely correlated phenomena. By causality, I am content with the following definition:

Connection between two events or states such that one produces or
  brings about the other; where one is the cause and the other its
  effect.

That is to say, an object experiences an acceleration because it is exposed to a net force; the force does not arise because of the acceleration. However, some philosophical thinking on Venturis has shaken my confidence in this idea. If the acceleration of the fluid through the constriction is caused by an unbalanced force, what causes the unbalanced force in the first place? Another way of asking the question is, how is the bounding geometry causally linked to the pressure distribution of the flow? My only answer as yet is that there's no other way to satisfy mass, momentum, and energy conservation simultaneously, but that seems decidedly unsatisfying. Is there any causality implied by Newton's 2nd Law?
 A: 
Is there any causality implied by Newton's 2nd Law?

It depends on what you mean by "causality" and "Newton's 2nd law".
I would rather not go into various meanings. Newton's original formulation of the 1st law seems causal:
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
But his formulation of the 2nd law does not:
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.*
Also, it is not necessary to formulate mechanics this way. The equation
$$
\mathbf F = m\mathbf a
$$
can be used without the verbal or causal formulation. In this equation, there is nothing referring to causality; it just puts two quantities into mathematical relation; whenever there is acceleration there is also the corresponding force. There is no delay and no subordinate relation between the two.
Jakov Frenkel made a very good observation that we think of force as cause because we see its accelerating effect only after the body acquires substantial speed, which happens after some time. But in fact, we believe that the acceleration is present exactly at the same moment the force is, so there is no delay between the two and no cause-effect relationship.
A: One of my favorite quotes, and I think this complements Ján Lalinský's answer:

"Does the engineer ever predict the acceleration of a given body from
  a knowledge of its mass and of the forces acting upon it? Of course.
  Does the chemist ever measure the mass of an atom by measuring its
  acceleration in a given field of force? Yes. Does the physicist ever
  determine the strength of a field by measuring the acceleration of a
  known mass in that field? Certainly. Why then, should any one of these
  roles be singled out as the role of Newton's second law of motion? The
  fact is that it has a variety of roles." 
  - Brian Ellis, The Origin and Nature of Newton's Laws of Motion (1961), as cited by A P French, Newtonian Mechanics. (a fantastic
  book)

$F=m\ddot{x}$ isn't a definition of force or a definition of mass, it's a relationship. 
As for your specific example, I can't help on the dynamics, but from the fact that the water accelerates, it must be pushed from behind (or pulled from the front, but you know with pressure the two are equivalent). This image on wikipedia:
http://en.wikipedia.org/wiki/File:Venturi.gif
in which high pressure is indicated by a dark blue color, gives you a pressure gradient. Clearly the change in pressure is enough to explain the acceleration/deceleration. Why is there a pressure gradient? That is deserving of its own question*.
*I'm answering the question "Is a causal relationship implied by Newton's 2nd Law?" 
A: What makes you believe that there us an unbalanced net force in a Venturi tube? The force required to change the momentum of the liquid comes from the walls of the tube. This can be easily observed with an open nozzle (a garden hose will do!) and is used in rocket motors. In closed form a U-shaped tube is used as one of the most precise flow sensors, which measures the flow of dense liquids by measuring the forces on the tube, see the Coriolis flow meter: http://en.wikipedia.org/wiki/Mass_flow_meter 
A: "what causes the unbalanced net force in the first place?" - potential. The energy of molecular motion upstream of the flow is greater than downstream. And we see the energy as pressure or potential (energy).
But regarding the main question - Is there causality wrapped up in Newton's 2nd law? Maybe. 
Consider F = d/dt(momentum). The fact that I had to write this expression using a derivative implies that there is prediction going on to figure out what force I have from the momentum of a system. On the other hand if I write a = integral(F/m), I'm only relying on the present and past states to determine motion.
I apologize - I'm a beginner and not skilled yet in properly formatting these posts.
