Bring 2 quarts of water to a boil. Add lemon juice and salt. Poach shrimp, uncovered for 2 to 3 minutes, depending on size, until cooked through.
Transfer shrimp to a bowl of ice water to stop cooking. Drain and pat dry. Remove heads, shells and tail. Coarsely chop shrimp and place in a bowl.
Peel mango and cut into 1/2-inch cubes. Mix mango with shrimp, basil and salad dressing.
Warm tortillas to soften. Divide watercress and shrimp mixture among tortillas. Roll up, tucking in sides to enclose filling. Cut wraps in half diagonally to serve. Serves 6.