Preheat the broiler and position a rack 6 inches from the heat. Toss the shrimp with 1 tablespoon of the olive oil and season with salt and pepper. Broil the shrimp for 3 minutes, turning once, until opaque throughout.
In a mini food processor, pulse the mint with the warm water. Add 4 tablespoons of the olive oil and puree until smooth.
In a medium bowl, toss the beans with the mint oil and shrimp and season with salt and pepper. In a large bowl, whisk the lemon juice with the remaining 1/2 tablespoon of olive oil. Add the watercress, season with salt and pepper and toss gently. Mound the watercress on large plates and spoon the shrimp salad on top. Serve with lemon wedges.