Cut tomatoes into wedges. Cut sweet pepper into small squares. Combine tomatoes, sweet pepper, zucchini, onion, and parsley; set aside.
For dressing, in a screw-top jar, combine oil, vinegar, water, thyme, and garlic. Cover; shake well. Pour over vegetable mixture. Toss lightly to coat.
Let mixture stand at room temperature 30 to 60 minutes, stirring occasionally. (Or, refrigerate, covered, for at least 4 hours or up to 24 hours, stirring once or twice. Let stand at room temperature about 30 minutes before serving.) Serve with a slotted spoon. Makes 6 to 8 side-dish servings.