Whisk together lime juice, sugar, salt, and pepper until sugar and salt are dissolved, then add oil in a steady stream, whisking until combined.
Combine lettuce, watercress, and half of pomegranate seeds in a large serving bowl. Toss with just enough dressing to coat, then sprinkle with almonds and remaining pomegranate seeds.
Cooks' notes: Lettuce and watercress can be washed and spun dry 1 day ahead and chilled separately, wrapped in dampened paper towels, in sealed plastic bags. Pomegranate can be seeded 1 day ahead. Chill seeds in an airtight container. Almonds can be toasted 1 day ahead and cooled completely, then kept in an airtight container at room temperature.