In a large bowl combine the 3 cups water, the cabbage, cucumber, sweet peppers, celery, onion, pimiento, and pickling salt; toss to mix thoroughly. Let stand 3 hours. Drain; rinse well with cold water. Drain again.
In a 4-quart kettle combine the vinegar, the 3/4 cup water, the sugar, and mustard seed. Bring to boiling; reduce heat. Simmer, uncovered, for 3 minutes. Add drained vegetables. Return to boiling; reduce heat. Cook, uncovered, for 5 minutes more.
Ladle hot relish into hot, sterilized half-pint jars, leaving a 1/2 inch head space. Wipe jar rims and adjust lids. Process jars in boiling-water canner for 10 minutes (start timing when water covering jars returns to boiling). Remove the jars from canner and cool jars on wire racks. Makes 7 half-pints.