Place the lemon juice, garlic, shallots, salt and pepper in a medium bowl, whisk to combine. Add olive oil in a steady stream while whisking to emulsify vinaigrette. Finish seasoning with salt and pepper to taste.
Place couscous in a large bowl. Combine water, harissa, parsley, cilantro, salt and pepper in a saucepan and bring to a boil. Pour the spiced, boiled water over the couscous and cover with saran wrap for 20 minutes. Fluff couscous with a fork.
Toss the crab, avocado, sumac, peppers, Serrano chili, couscous, and 1/2 cup lemon vinaigrette in a large bowl and season with salt and pepper to taste.
In a separate bowl, toss the watercress and julienned tomatoes with a 1/2 cup of the lemon vinaigrette.
Place 1/2 cup Sumac Couscous Salad in the center of a plate. Mound about 1/3 cup of the watercress mixture on top of the couscous. Serve immediately.