1. Wash, peel and cut the potatoes into pieces. Steam or boil them for 20 to 30 minutes. Store in a cool place.
2. Mix the olive oil and balsamic vinegar in a bowl. Season to taste.
3. Remove the stone from the avocados, then peel them and cut them into pieces. Peel the grapefruit using a knife and separate the segments.
4. Mix the potatoes, shrimp, avocado and grapefruit. Add the vinaigrette.
5. Serve chilled.

Tip: For a tangy touch add a few pomegranate seeds.