I do not know if anyone has posted about this kind of thing before; if they have, my apologies. The same goes if this is in the wrong place; I am new here, if not to DF.
This only really works when dealing with very large amounts of water, such as an aquifer or a river; I have found the design quite useful for draining flooded fortresses, as well as for creating needlessly massive waterfalls.
The basic layout for a floor is:
W: Wall
F: Floor
=: Water wheel
%: Screw pump (always pumps from direction of water wheel)
*: Gear assembly
-: Horizontal axle EW
|: Horizontal axle NS
WWWWWWWW
===%%FFF
W*-*WWWW
WWWWWWWW
The orientation is arbitrary; so long as the water wheel sits over the previous floor's pump output, it will function perfectly. An increased amount of pumps and water wheels allows for increased water movement and power generation, but takes up more space; this is the most compact version.
The pump assemblies can be built over the lower pump assemblies to allow power to move up levels. To use this as a generator, attaching another gear assembly in place of a horizontal axle on the floor in question works perfectly.
WW|WWWWW
W***WWWW
===%%FFF
WWWWWWWW
This can easily be run in parallel, which moves water faster and generates substantially more power, although not more than a stack.
Where N is the number of parallel systems, the net power produced comes out to: 90*N (+90 per water wheel) - 10*N (-10 per screw pump) - 3*5 (two gear assemblies, and a third to move the power out) - 1 (axle) = 80*N-16. Without the gear assembly to remove the power, a floor produces 80*N-11 instead.
Turning the assembly from the E-W orientation shown above to a N-S orientation is easy (the process can be reversed from this without too much thought). Make sure to expand the N value of the N-S device.
WWWWWWW
WWWWWFW
W*-*WFW
===%%FW
WWWWWWW
That is the extent of my information; I have a few more complicated structures that I designed but they do little but illustrate the principle. If screens are requested, I can provide them; I have not taken any captures at present but the device is not about to break down.