I have not tried the fill pond strategy to make a farm, but rather the "planned flood" technique.
In 2d...
+---!---+ RRRR
|fffffff| RRRR
|fffffff| +----+______RRRR
|fffffff|X|wwww?__C__CRRRR
|fffffff| +----+ RRRR
+-------+ RRRR
Where:
- R is a river (with 1+ layers of water above it, for pressure)
- w is water storage, f is farm (7 f to 1 w)
- C is a channel from 1 square above
- X is a wall to dig out
- ? is a door/floodgate hooked to a switch somewhere.
- ! is a door for mud/water containment
To make the water storage:
- dig left C, tunnel, and storage room
- build door/floodgate at ?, and connect it to a switch
- make sure ? is OPEN
- Channel out right C
- When room is 7 deep, flip switch
Now, the storage room is filled with 7 * w units of water, one unit per bit of farm!
To make your farm muddy:
- Channel at X
- Run out of door
- Lock door until room is full of mud (and possibly fish)
If you're dead-set on making due with this embark area, I would dig for the caverns. Pretty much just pick a point, and dig straight down. (If you grabbed the newest release, this has apparently been made easier by allowing designations that span z-levels.)
If you're just digging a small stairway, and not an elaborate 'grand central vein' as I usually do, it should be relatively fast to hit the caverns. I'd suggest having two dwarves dig out two side-by-side up/down stairs. This is more efficient than a single stair because they won't fight over who is digging where/when.
Once you hit the caverns, the first thing you see is almost always an underground lake. The second thing you almost always see is underground creatures. You may want to just grab a few buckets of water quickly, then floor up your stairs to keep them from surfacing, at least until you have a militia.
Once you have a fertile farmland, you never need to water it again, and can have infinite booze. You will, however, need water for tending wounded, so it might be worth building a well near the underground cavern. The bucket can drop down from the cavern ceiling, protecting you from all but flying creatures. Don't worry if the well is a hike - water is hopefully seldom used after farms are running.
Best Answer
Tunnels. dig tunnels to drain them all into an underground cistern. You have two concerns here, pressure and stagnancy. Pressure can be stopped using a diagonal gap, the way I fix stagnant water is with a pump, but the wiki page says that mixing it with clean water will also work.
I would suggest a system where one joins all the pipes together, then passes the water through a diagonal gap to a pump. The pump can be powered by a dwarf, or a waterwheel. If you put the waterwheel after the pump in the water's path then you can start draining the pools by telling a dwarf to pump, and then it will be self sustaining, until it runs out of water. After it stops though it won't start up by itself, so you won't have to worry about random fortress flooding when you aren't on the lookout for it.
My suggestion would look like this:
Where the Os are solid walls, the blank spaces are tunnels, the %% is the pump, which is facing east(right), the _ are channels, the === is a waterwheel, built over channels, the * are gear assemblies, and the -- are axles. The little gap in the upper left corner is the tunnel coming in from your pools. I have drawn it as though it were full of empty space, but digging it out of up-down stairs is likely much easier and will not impede the water in any way.
There is one other thing you will want to be aware of, although I am not sure how much it affect the current version. Once water has passed through your tunnels they will have muddy ground, and in some circumstances will start growing things, including trees. To avoid the mess of plants growing in your pipes, consider digging all the water channels with up stairs instead of just floor. (As long as you don't dig any down stairs, up-down stairs, or channels in the tiles above these tunnels the ceiling will remain intact.)
Edit: One thing I forgot to mention is that an important consideration in laying out your tunnels is that long horizontal channels are bad in two ways.
They slow down water movement.
They lose water to evaporation more readily.
This means that your cistern should be close to your purifier, and that you should build both of them close to the center of your water collections. Also, joining tunnels together early could potentially help with evaporation issues.