Combine everything for the dressing, apart from the herbs, with ¼ teaspoon salt in a food processor or blender.
Purée until smooth, scraping down the sides as needed; add the herbs and pulse several times until combined. Pour into a serving bowl; cover and chill until ready to serve.
Remove the quinoa from the heat when ready. Let cool for 5 minutes, still covered.
Fluff the quinoa with a fork to separate the grains. Transfer to a large mixing bowl; add the tomatoes, cucumber, pomegranate seeds, watercress, and a pinch of salt and pepper to taste. Toss to combine.
Spoon the salad onto a platter, placing the fish next to it. Spoon over some of the dressing; serve with the remainder on the side.