Heat a large straight-sided skillet over medium-high; swirl in oil. Add onion and 1/2 teaspoon salt; cook, stirring occasionally, until onion is translucent, about 3 minutes. Stir in curry paste and cook 30 seconds. Stir in coconut milk, 1 cup water, and another 1/2 teaspoon salt. Squeeze tomatoes' juices into curry mixture, then drop in tomatoes and boil 3 minutes. Stir in couscous and peas. Scatter shrimp on top, in a single layer; return to a boil. Cover, reduce heat to medium, and simmer until shrimp are opaque and just cooked through, 5 to 7 minutes. Serve, with basil scattered on top and drizzled with oil.