I found moats to be unproductive. It's hard to hit mobs from the outside, and when I go out to cull creepers in the morning it's better when they are spread out. A bit more walking to collect the bones and rotten flesh perhaps, but that's good enough at the beginning and later on I can just build the trap I talked about a couple of pages ago (short version - arrange to be able to hit them in the legs through a half-block gap).
That's because you didn't put 1x1x3 pits at the corners/ends of your moat.