Combine first 5 ingredients. Cover and chill.
Combine water and next 4 ingredients in a large skillet; bring to a boil over medium heat. Cover, reduce heat, and simmer 10 minutes. Add salmon to skillet; cover and simmer 10 minutes.
Remove skillet from heat; let stand, covered, 8 minutes. Remove fish from skillet. Discard liquid and vegetables.
Serve with horseradish mixture. If desired, garnish with lemon wedges and celery leaves.