In a large, non-reactive container, whisk together water, soy sauce, apple cider vinegar, rum, brown sugar, salt and pepper until dissolved. Add onions, garlic, ginger; stir to combine. Add salmon fillets, making sure they are covered with brine. Cover the container and marinate overnight in the refrigerator.
The next day, remove fillets from the brine, pat dry with paper towels, and arrange on racks.
Smoke salmon according to your smoker’s instruction manual. We like to use a combination of hickory and alder chips. Check fish frequently for doneness.
Cook's Note: Do not reuse brine, always make a fresh batch.