Bring black-eyed peas and water to a simmer in a saucepan. Cover and reduce heat to low, cooking until they are fork-tender, about 20–30 minutes. Drain and rinse in cool water. Place in a covered container in refrigerator until ready to serve.
Meanwhile, whisk together the rosemary, olive oil and vinegar. Season generously with salt and pepper.
When you're ready to serve, toss the black-eyed peas with the dressing.