Cut corn from cobs. Measure out 8 cups of corn in a 10 quart Dutch oven pot. Combine corn and water. Bring to boil and reduce heat. Simmer, covered for 5 minutes. Drain corn. In same Dutch oven combine with corn, red and green peppers and onions. Stir in vinegar, sugar, mustard, salt and black pepper. Bring to a boil. Boil gently, uncovered for 5 minutes, stirring occasionally. Stir together cornstarch and 2 tablespoons water. Add this mixture to corn mixture. Cook and stir until slightly thickened and bubbly. Cook for 2 minutes more. Ladle relish into hot sterilized jars leaving 1/2-inch headspace. Remove any air bubbles. Wipe jar rims and adjust lids and rings. Process in hot boiling water canner for 15 minutes.