In a medium skillet with a tight-fitting lid, boil the chicken broth over high heat until reduced by half, about 15 minutes. Reduce to a simmer and add the lime zest, lime juice, coconut milk and basil. Place the salmon, skin side up, in the simmering broth, cover and poach over low heat until opaque, about 10 minutes.
Meanwhile, heat a medium nonstick skillet over high heat. Add the watercress and cook, tossing, until wilted, 1 to 2 minutes; drain and season to taste with salt and pepper.
Transfer the cooked salmon fillets to serving plates. Remove the lime zest and bring the poaching liquid to a boil. In a small bowl, dissolve the cornstarch in 2 tablespoons of cold water and whisk into the boiling liquid. Lower the heat and continue whisking until the coconut sauce thickens, about 1 minute. Spoon some sauce over each fillet and serve with the wilted watercress and lime slices.