Heat the water, lemon, peppercorns and coriander seeds in a 3-quart saucepan over medium heat to a simmer. Add the shrimp and cook for 2 minutes or until the shrimp are cooked through. Rinse the shrimp mixture with cold water and drain well in a colander. Discard the lemon, peppercorns and coriander seeds.
Stir the mayonnaise, chipotle pureé, lime juice, celery and onion in a large bowl. Add the shrimp and cilantro and stir to coat. Cover and refrigerate for 30 minutes.
Divide the bacon, shrimp mixture, avocado and watercress among 4 bread slices and top with the remaining bread slices.
Easy Substitution: Substitute low fat or canola mayonnaise for the mayonnaise.