Your moat as it stands is fine, so long as it is a ditch, and not actually filled with fluids.
The game functionally treats fortifications as floor tiles that are unpathable. (Unless 7/7 fluids are present.) If something manages to get into a fortification, it can just walk out, but nothing is capable of recognizing fortifications as something they can walk into, and they cannot be climbed.
It IS, however, reported that vertically diagonal movement can occur if the overhang is fortificaitons. (That is, moving from a wall to an open floor one tile above and horizontally one tile away when a fortification was directly overhead.)
You should probably also make your moat one tile wider and put a drawbridge in there as a "shutter" to stop goblins from firing in if you go with fortifications. You don't want elite marksgoblins making swiss cheese of your dwarves, as elite-level ranged combatants suffer no penalty for firing through fortifications, and WILL shred dwarves depending on fortifications for protection unless they are armored heavily enough to deflect the arrows.
Also, keep in mind moats are totally useless against fliers. Personally, I forgo moats entirely in favor of just excavating defenses below the surface, and having "shortcut" exits covered by drawbridge when threats come knocking, "trader" entrances that are shut off from my fort, and only path to the trading depot (which makes them unattractive to siegers), and then "sieger" entrances that are filled with deathtraps.