In one of my many fits of boredom on the isolated fort, i decided to build a power station, powered by the local brook, without breaching the surface. I ended up with a setup that works, in spite of being not really logical. Basically, i tapped the brook near the source, and once again close to where it leaves the map, connected the two with a channel and let the flow drive my waterwheels. However, since i had vowed to never breach the surface, i moved the channel driving the wheels one z-level down. ...and then it goes back up and re-joins the river.
Basically, it looks like this:
________________________________________
~+~~~~~__WWW.WWW.WWW.___~~~~+~
--------\~~~~~~~~~~~~~~~~~~~~/------
--------------------------------
~ - Water
- - Floor
_ - Ceiling/undug space above
WWW - one waterwheel, drawing power from the 'flowing' water below
+ - Fortification as inlet/outlet
The closed ceiling makes sure the middle area doesn't flood. It's a bit weird that there's usable flow there, because it should just try to flow down from both sides until full and then stop. What's also weird is that those waterwheels don't produce the standard 100 power, but only 50 each. I started with four, which gave 200 power, and later added another four, which increased it to 400.
(P.S. So, Verdana is the monospace font? I should probably write this down somewhere.)