In the hydraulic analogy to electrical current, pressure is voltage, water moving is the current, but what is a resistor? I've heard that a resistor corresponds to a constriction in the pipe, but a series of constrictions is not additive like a series of resistors, right? Can anyone give or cite a better example of the hydraulic analog to a resistor?
In basic physics concepts, which relate to fluid movement, the cross-sectional area of the pipe/medium/whatever the fluid flows through determines the flow rate, for the given pressure.
As you say, if pressure is "voltage", and velocity of water is "current" (or the movement of electron charge in electricity, Coulombs per second), then indeed the restriction of the current is given by the "pipe" that it moves through. More pressure (voltage) will induce more water movement (current) through the given aperture's cross sectional area (effectively resistance).
edit: I should mention that pressure drops along the length of pipes, and that if the pipe is not 'full' of water, then it's quite more complicated to work out the area of water moving through it. The velocity of water and the "flow rate" are also a little bit different, you should try to find a few simple equations for whatever application you are doing. There are some online calculators you can use, in most cases.
Water analogies are always fun. I've always considered a resistor like a big, porous sponge obstructing the pipe :)
One place the use of water analogies breaks down is that there is no variable "speed" of current flow. If you are talking about energy transfer, then the velocity is simply the speed of light (in whatever medium you're using). If, instead, you are talking about electron movement, it's so slow that it becomes insignificant. The actual electron drift in a 1-mm thick copper conductor is on the order of one millimeter per second!
The other place it breaks down is that there is no inertia of current. You can add inductance, however, to bring about a similar effect.
There's an excellent example of this analog. Check out this Youtube of a Ram Pump. It's a neat, old technology that many people have never seen. It turns out that it's exactly the same as a boost converter! You use a diode instead of a one-way valve, a capacitor instead of the pressure tank, a MOSFET instead of the oscillating valve, and add inductance to make the inertia work out.
Have fun :)