Vietnamese Tuna Seviche

Name: Vietnamese Tuna Seviche
Ingredients: 3 tablespoons Asian fish sauce (nuoc nam); 2 1/2 tablespoons sugar, plus more for sprinkling; 1 1/2 tablespoons water; 1 tablespoon fresh lemon juice; 1 tablespoon fresh lime juice; 1 small garlic clove, minced; 1/2 teaspoon minced fresh ginger; 1 serrano or Thai chile, thinly sliced, with seeds; 1 cup yellow cherry tomatoes, halved; 1 tablespoon soy sauce; 1 tablespoon rice vinegar; 1/2 pound cold sushi-grade tuna, cut into 1/2-inch cubes; 1/2 pound yellow seedless watermelon, cut into 1/2-inch cubes; 2 tablespoons tobiko (flying-fish roe); 2 tablespoons torn purple or green basil leaves