In a small saucepan, combine the water with the vinegar, salt and sugar and bring to a boil, stirring. Let the pickling liquid cool to room temperature, then add the pepper, rosemary and garlic.
Layer the sliced tomatoes in a 1-quart glass jar and cover with the pickling liquid. Let stand at room temperature for 3 hours, then store in the refrigerator. Drain the pickles before serving.