In a medium bowl, whisk together the soy sauce, olive oil, water, rice vinegar, sugar, Dijon mustard, and grated garlic.
Place the salmon in a large dish or container skin side up, and pour in the marinade. Marinade for at least 30 minutes at room temperature (or up to 3 hours refrigerated; if so, allow to sit at room temperature 30 minutes before cooking).
After marinating, place the salmon on a baking sheet, making sure to tap off excess marinade before cooking if you are pan searing. Brush it liberally with the marinade if you are grilling, then discard the remaining marinade. Sprinkle with the kosher salt (split between the fillets) and fresh ground pepper.