Combine the water and 1⁄2 teaspoon salt in a bowl and microwave for 30 seconds. Stir so the salt thoroughly dissolves, then mix in the vinegar, parsley, garlic, and pepper flakes.
Slowly drizzle in 2 tablespoons of the olive oil, whisking to incorporate. You can use the chimichurri now, but it's best to let the flavors marry for 20 minutes or more; it will keep covered in the fridge for 3 days.
Heat the remaining 1 tablespoon oil in a large skillet over medium-high heat. Thoroughly dry the scallops with paper towels, then season on both sides with salt and pepper.
When the oil is hot, add the scallops and cook for 2 to 3 minutes on the first side, without disturbing them, until a deep brown crust has developed. Flip and cook for 1 to 2 minutes longer, until firm but yielding to the touch. Serve drizzled with the chimichurri.
