This is probably old news to people interested in nonstandard methods of computation, but it just occurred to me that the fluid-based analogy to transistors is straightforward to construct:

              S
            |   |
     -------|   |
   G  \/\/|##|  |
     -------|   |
            |   |
              D

This is essentially a pressure regulator, except that the pressure is controlled by an independent signal. Pressure in G pushes a spring-loaded piston to block flow from S to D (a slightly different construction instead allows flow when G has pressure). Modulating the pressure in G can also used to modulate the flow, based on F = -kx. This simple construction has some issues, such as the fact that the pressure needed to move the piston depends on the pressure in S-D.