What corresponds to a resistor in the hydraulic analogy to current?
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.