How to Make It
Combine water and 2 tablespoons of the salt in a large saucepan; bring to a boil over high. Remove from heat. Add shrimp; cover and let stand until shrimp curl and turn pink, about 3 minutes. Remove shrimp from liquid, and cool on a wire rack on a baking sheet, about 10 minutes.
Meanwhile, combine potatoes, 2 tablespoons of the salt, and water to cover in a medium saucepan. Bring to a boil over high; reduce heat to medium-low, and cook until potatoes are fork tender, about 10 minutes. Drain and cool 10 minutes.
Whisk together Champagne vinegar, lemon juice, garlic, shallot, herbs, celery seeds, mustard seeds, and remaining 1/4 teaspoon salt. Gradually add oil, whisking constantly.
Toss shrimp with 2 tablespoons of dressing; let stand 10 minutes. Toss peaches with 2 tablespoons of dressing; let stand 10 minutes.
Combine potatoes, romaine, and radishes in a large bowl. Toss with 2 tablespoons of dressing. Stir in shrimp and peaches. Divide mixture evenly among 6 plates; sprinkle with pecans. Drizzle with remaining dressing, and serve immediately.