In a small saucepan, bring water to a boil. With a sharp knife, cut a small x on the blossom end of the tomatoes. Blanch tomatoes for 45 seconds, then rinse with cold water. (This makes them easy to peel. Alternately, leave the skins on.) Peel and dice tomatoes.
In a bowl, combine all ingredients and mix thoroughly. Let stand at room temperature for about 1 hour to blend flavors.