Put olive oil, lemon zest, and mustard in a small bowl and stir to combine. Put salmon in an ovenproof baking pan in which it fits in a single layer without overlapping. Spread olive oil mixture evenly over both sides of fillets. Cover and refrigerate for 20 minutes.
Preheat oven to 400°.
Remove salmon from refrigerator and add water. Sprinkle salt over fillets and season with pepper. Bake just until an instant-read thermometer registers 127°, approximately 7 to 9 minutes depending on the thickness of the fillets. Salmon should be opaque and beginning to flake.