Preheat oven to 350 degrees. Spread pepitas on a rimmed baking sheet; toast until golden brown, 10 to 15 minutes. (Store at room temperature up to 1 day.)
In a large bowl, combine vinegar and cumin; whisk in oil. Season with salt and pepper. (To store, cover and refrigerate up to 2 days.) Add watercress; toss to combine. Serve topped with pepitas.