This appetizer is "cooked" without heat; the acid from the citrus juices makes the shrimp firm and opaque.
Slice off top and bottom of grapefruits with a chef's or paring knife. Cut away peel and pith in wide strips by running blade from top to bottom of fruit, following the fruit's curve. Holding fruit over a bowl (to catch juices), cut each segment on both sides to free from membrane, allowing segments to drop into bowl.
Bring vinegar and onion to a boil in a small saucepan over medium heat. Reduce heat to low, and cook 5 minutes. Let cool completely. Strain, and discard liquid. Transfer onion to a small nonreactive bowl, and refrigerate until ready to use.
Toss together shrimp, grapefruit segments and juice, the lime juice, salt, and sugar in a nonreactive medium bowl. Refrigerate until shrimp are firm and opaque, at least 4 hours (up to overnight).
Divide grapefruit segments among bowls; top with shrimp and then onion. Sprinkle with sea salt and pepper.