Place the couscous in a large bowl. Add 1 cup very hot tap water and ¼ teaspoon each salt and pepper. Cover and let sit for 5 minutes; fluff with a fork.
Meanwhile, in a medium bowl, combine the shrimp, snap peas, tomatoes, basil, scallions, oil, lemon zest and juice, ¾ teaspoon salt, and ¼ teaspoon pepper and toss to combine. Serve over the couscous.