Bring a large pot of water to a boil. Add salt and pasta. Cook until al dente according to package instructions, about 12 minutes.
While pasta is cooking, heat oil over medium-high heat in a medium skillet. Add garlic and white parts of scallions, and cook until fragrant, about 30 seconds.
Add shrimp, and cook 1 minute. Turn and cook until pink and firm, about 1 minute more. Stir in peas, lemon zest, and red pepper flakes. Add 1/2 cup pasta cooking water, and cook until peas are bright green and shrimp are cooked through, about 1 minute more. Transfer to a large bowl.
Drain pasta and add to bowl with shrimp. Add watercress and mix gently. Drizzle with lemon juice. Season with salt and pepper, and toss to combine. Serve immediately, garnished with scallion greens.