Well you can't tunnel off the side of the map because the edge tile is never diggable, so that's out anyway. So it turns out you can turn an edge tile into fortifications which will drain water. First smooth it, then carve fortifications. (Thanks Nakedible!)
However, for those who consider this cheating or who are using a non-infinite water source, the only way is, indeed, a pump stack. This, while time consuming, isn't as terrible as you might think. (You'll probably have to do this later to bring magma up from the deeps, anyway.) The wiki has a description of optimal pump stack construction.
This will allow you to build a stack of mechanically operated pumps without putting extra machinery on each level. You only have to power one of the pumps in the stack. (Assuming that power source provides enough power for the whole stack.) Combine this with the (somewhat cheating) dwarven water-wheel reactor and you have a relatively painless pumping solution.
I like to put two gears between the power source and the pump stack and attach a lever to the gear nearest the pump stack. This allows me to turn the whole thing off if chaos ensues for any reason or I need to do maintenance to the system.
The alternative is of course a genuine water wheel setup or windmill setups, but these options will force you to be more choosy about where you locate the construction. Any axles or gears that lead from your power source to the pump stack consume power!
You might want to try building a smaller pump stack before going for one that spans 50 levels or more. Keep in mind it's very easy to flood your fortress with Fun!
I know it's not much of an answer, but the method I usually use is to make sure there's an up stair at the end of the tunnel, dig out the other end, have the dwarf run up, away from the water and then building a floor on the Z-level above it to cover the hole.
Dwarves are stupid. That's just a game mechanic.
Best Answer
For moving water down, the right way to do that is with pumps or a large reservoir* that will not overflow (if the lake is 20 tiles then your reservoir must be at least 20 tiles at or below the level you wish the water to be at). To move water up, you will need a pump stack. To move water horizontally just build a tunnel (digging it 1 level below the water will help with the speed, due to pressure. You can also use buckets - designate a pond activity zone over a downward shaft and have dwarves carry water there.
Note that still water can be stagnant, in which case it will lead to unhappy thoughts and infections (while cleaning wounds), you can purify stale water by pumping it.
You really shouldn't be shy of using pumps - they only require 2 units of wood and 1 unit of stone to build and can be operated manually if you don't have power.
If you are worried about pressure and don't want extra pumps, there is a bug you can exploit - pressure does not travel diagonally, so you can just dig a shaft from the lake to the level you need the water at, then make a diagonal tunnel for water to travel through - it will not flow up. Here's an a top-down example.
Not all sources of water have pressure. Only rivers source tiles, screw pumps and falling water into more are pressurised. So you only need the above cheat if one of these three sources of pressure occurs.
*Obviously a reservoir is not going to work if you are dealing with an infinite water source such as a river or an aquifer.
Find out more about water pressure.