1. Combine lime juice, fish sauce, cilantro, mint, and ginger in a large bowl.
2. Split shrimp in half lengthwise, then add to bowl and gently toss with dressing to coat.
3. Spoon grapefruit segments into a wide, shallow serving bowl, using a slotted spoon. Pour 1/4 cup grapefruit juice into bowl with shrimp. Add sambal to taste, gently mixing to combine (save remaining grapefruit juice for another use).
4. Spoon shrimp and dressing onto fruit. Add watercress and gently toss.
*Find sambal badjak, a sweet-spicy Indonesian condiment, in the international foods aisle.
Note: Nutritional analysis is per serving.