In a large bowl, combine the tomatoes, bell pepper and onion. Toss to mix.
Mix together the vinegar, salt, mustard seed, celery seed, sugar, cayenne pepper, black pepper and water in a small saucepan and bring to a boil. After boiling for 1-2 minutes, pour the mixture over the tomato / pepper / onion mixture.
Once cool, place in a tightly covered dish and refrigerate for at least 4 hours before serving. Will store up to 48 hours refrigerated.