Prepare an ice-water bath; set aside. Bring a medium pot of water to a boil; add salt. Blanch green beans until crisp-tender, 4 to 5 minutes. Immediately transfer with a slotted spoon to ice-water bath to stop the cooking. Drain, and set aside.
Whisk together shallot, lemon juice, and 1/2 teaspoon salt in a large serving bowl; season with pepper. Whisking constantly, pour in oil in a slow, steady stream; whisk until emulsified.
Add cucumber and reserved green beans to bowl; toss to coat. Add watercress and mint; gently toss. Season with salt and pepper.