Cut off and discard head of octopus, then cut tentacles into 1-inch pieces. Generously cover octopus with water in a heavy medium pot and gently simmer, uncovered, until tender, 45 minutes to 1 hour. Drain octopus in a colander and cool to room temperature, then transfer to a bowl. Stir in remaining ingredients, 1/2 teaspoon pepper, and additional sea salt to taste. Let stand 30 minutes for flavors to develop.