Sprinkle bread with water; let stand 2 minutes. Carefully squeeze moisture from bread. Tear into 1-inch pieces. Let stand on paper towels 20 minutes.
Combine vinegar, oil, salt, and pepper, stirring with a whisk. Combine ricotta and next 5 ingredients (ricotta through thyme) in a large bowl. Add bread, tomatoes, and onion to ricotta mixture. Drizzle with vinaigrette; toss gently to coat.