Also, some few creatures can swim.
A double-deep moat with only 1 level of water will solve both these problems, and still look cool. (Because that's the only reason to fill a moat with water in DF.)
ZH - re pumping or channeling for the water, think ahead.
You're going to dig the ditch - okay, there, let's say it's dug. Nice ditch, perfect defense.
But if you want to fill it with water, you can either use pumps or channel out an opening to some water source - a large pond or a river, whatever.
If you do the latter, just dig it out, it's very difficult to block that water flow off if you ever need to.
Pumping gives you good control - you can add "some" water, and let your dwarves wade in it if/when they need to - you get the look, and still have some access. You could reverse the process and pump it dry if you needed to do some remodeling.
If you're just going to channel, at least add some sort of water-control gate linked to a lever - a floodgate, a door, a 2x2 raising bridge (when it raises, it acts as a wall). (This last is my preference, because nothing will jam it open!) You fill, you close, you can pump dry later or add more or whatever.
If you simply channel, you're stuck. It will work, but you're stuck with it.