Pour the corn into a colander and rinse thoroughly with cool water to defrost.
Drain well, and place the corn into a 2-quart or larger bowl.
Peel and finely chop the onion, and add it to the bowl.
Coarsely chop the zucchini and bell pepper, and add them to the bowl.
Add the chopped cilantro leaves.
In a 2-cup glass measure, microwave the water and sugar, uncovered on High, for 45 seconds.
Whisk the mixture until the sugar dissolves.
Whisk in the oil, vinegar and black pepper.
Salt to taste, if desired.
Pour the dressing over the vegetables, and stir to coat the vegetables well.
Serve at once, or refrigerate until ready to serve.