In a large skillet over medium heat, combine chicken and water; cook, stirring, until just warmed through (3–5 min). Stir in chili powder, 1 teaspoon lime juice, and ¼ teaspoon each cumin and pepper until the chicken is evenly seasoned.
Meanwhile, in a small bowl mix tomatoes, onion, serrano peppers, cilantro, salt, and the remaining 3 teaspoons lime juice plus ¼ teaspoon each cumin and pepper to make pico de gallo.
To serve, arrange tortillas on a platter. Divide chicken and pico among tortillas; top with lettuce, cheese, and sour cream if using. Garnish with extra cilantro and serve with lime wedges.
